My FICO 8 score according to Amex is almost 100 points lower than the FICO 8 score contained on Myfico.com. Does anyone have any insight or experience with this anomaly? I don't understand why Amex would have such a different score. Thank you.

American Express provides access to at least two credit scores. Which are you looking at?

One is accessed at Account Services > View FICO Score

Another is accessed at https://www.mycreditguide.americanexpress.com/

There are a number of potential reasons. Are they pulling from the same bureau? AMEX pulls from Experian. Also, AMEX only pulls once per month. If your score has changed since AMEX pulled, there will be a discrepancy.

Yes, I see the scores from Experian and the other 2 agencies. Amex claims to pull a recently updated score which does not correspond to my current or recent scores.

I've often seen pretty large discrepancies depending on where it's pulled but 100 points seems extreme. There have been some changes within the last few years and it seems all credit scores are not created equally. I recently checked from a link one of my cards was offering and it came back at 835 but on my Identityguard monitoring I was just in the high 700s for each bureau. Still, 100 seems off unless you've recently made some substantial changes that haven't been picked up by everyone.

FICO 8 Experian, monthly score.

VantageScore� 3.0 TransUnion, weekly score (ignore it) and credit report.

The third option is CreditSecure for $16.99, providing you with FICO 8 scores (same as myFICO) and reports for all three credit bureaus. Thanks for the reminder by the way, I keep forgetting to pull them, as my scores are always the same due to nothing happening in my life. They are not updated automatically, you have to do it manually. A little like insurance I guess, you don�t necessarily use it all the time, but it�s good to have it when you unexpectedly need to.

You�re right. That is extreme. It can happen of course, if say a maxed card or a late comes around or drops off. Or maybe if a really old closed card falls off. But not from applying for a card or something like that.

Each credit bureau has a unique FICO algorithm. One may prioritize debt, inquiries, credit lines in a way that another doesn�t. That�s one thing. The other is that the data may differ. Occasionally, you may see an item on one report and not on another, which depends how the lender reports it, it�s a convention, but not mandatory. Or you may see an item being listed as revolving on one, but in another category on another. Such differences happen far less than they don�t.
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(Hope you're enjoying this morning's lightning/thunder/rain show!)

If you pull same credit bureau Amex/myFICO FICO 8 scores at the same time, they will be identical. A lender, such as a credit card company or a bank, giving you free monthly scores on the other hand may have some date discrepancy. One bank that I get a free FICO score from says the next time your score will be pulled is the 15th, but it�s generally somewhere from 3rd to the 10th, and I�m not sure the date that the score provides is really when the score was pulled, some batch processing may play in here.

For the sake of certainty, which of the three Amex offerings are you using?

I am using the score accessible from "Account Services." It is from 8 days ago but 96 points lower than my updated MYFICO 8 score, current as of 4 days ago. I haven't had any major credit shocks or changes so I remain at a loss to understand this discrepancy.

I do not have a MyFICO subscription, but I checked ms mia's FICO score at American Express (Experian updated 8.12.2020) and Barclays (TransUnion updated 8.09.2020), and even though they are from different bureaus they are within 7 points of each other. Can you check your FICO score elsewhere to judge which is the outlier? Which MyFICO plan did you purchase?

You would generally obtain a myFICO score in one of two ways: 1) you go to myFICO and purchase a report or 2) you have a subscription, which sends you alerts with an updated score. Which one applies to you? Either approach would inform you of the credit bureau. Does it mention Equifax, Transunion or Experian in such a report or alert of yours?

As soon as the scores are not pulled simultaneously, they will have some differences.

But for 100 points, I will check the credit report just in case.

A 100 point discrepancy would not shock me if it was one of the bogus Vantage 3 numbers that many of the "free" services are based on. I currently show a 63 point gap (And yes, I know OP has said he's looking at two FICO 8 scores.)
You can also purchase a subscription/monitoring through Experian at I believe $24.99 per month. That gives you daily Experian data and pulls from TU and Equifax every thirty days. It also shows you your Experian score under older FICO models, which some lenders may still use.
Wild!

With all the CC companies now offering some level of "free" FICO scores my score bounces up and down like a yoyo. My paid for Amex Credit Secure seems the most reliable and consistent with some changes monthly but not drastic. The scores from Chase or Citi can easily be 50 or more points different. Worse thing is I hust applied for a refi since the rates are so so low - I got an immediate 30 point hit on Credit Secure.....

I have a subscription to myFICO.com which provides me with both alerts and annual reports. Nothing in my recent myFICO data closely resembles the lower score found on my Amex account. MY myFICO account lists all 3 agencies in its FICO 8 scores.
Very bizarre situation especially since my Citibank Prestige account shows a much higher FICO 8 score, albeit from a different agency.

Citi is providing a version that uses a different scale: Your FICO� Score is calculated based on data from Equifax using the FICO� Bankcard Score 8 model and is the same score we use, among other information, to manage your account. This model has a FICO� Score range from 250 through 900.

American Express: Your FICO� Score 8 based on Experian data is the same score used by American Express. Your credit score summarizes information on your credit report into a single number that lenders can use to quickly assess your credit risk. Your FICO� Score is a three-digit number calculated from the information on your credit report that falls within a 300-850 range. Your Experian credit report is used to calculate the score displayed above; as information on your credit report changes so will your credit score.

Very mysterious. I wonder if it's an Amex hiccup and what next month will show. For now, I would trust myFICO.