CreditKarma pulls from TransUnion. It is very likely that TransUnion's data (and thus CreditKarma's score) are different from scores which may be reported to you by Experian or Equifax. Multiple reasons why:

1. The bureaus may have different data from your creditors. For example, a creditor might report your account only to one of the three bureaus. This is rare (usually they report to all three) but it does happen. This will cause differences in scores across credit bureaus.
2. The credit bureaus get updates on your accounts at different times. Even on one account, you will probably see different balances depending on when the credit bureaus were last updated with your most recent payments.
3. Inquiries will be different. When pulling your credit, unless it's for something major (such as a mortgage), creditors typically only pull from one bureau. The number of inquiries affects your credit score, so this is another reason why it will vary from bureau to bureau.
4. The bureaus have different scoring models. FICO, TransRisk, etc. Thus when you are comparing scores, you may not be comparing apples to apples. A newcomer to the game is the VantageScore, which is intended to make scores comparable across bureaus.

This is why CreditKarma exactly matching the Experian score is actually a coincidence rather than something that should be expected.