One more thing you can do is log on to your AMEX account and look at your credit report. It's under the "Account Services" tab, then look for the FICO score link (mine is under "Find it Fast"). You'll get a FICO score and then below you can expand out your credit utilization report. If all your other cards show there then they're using the right file. Keep in mind there are three major credit bureaus so you really need to check all three. Credit Karma will show you TransUnion and Equifax while AMEX is showing me Experian and I presume they'll do the same for you.

It is possible that the credit file that the AMEX tool shows you is different than the one(s) they are actually using for credit determination. AMEX may not have done a hard pull on you for a very long time. Once you check all 3 bureaus as I suggest, it's time to give them a call.