Your credit scores are calculated based on information your creditors send to the credit bureaus. But the way that your score is calculated depends on whether you or your lender requests it.
Each credit bureau has a consumer credit score that they market to the general public. Equifax has their Score Power score, TransUnion calls thes the TransRisk New Account Credit Score while Experian markets the PLUS score. Using the same credit history for developing your consumer credit score, each credit bureau also calculates a separate score. These are supply to lenders making an inquiry.
Your consumer credit scores are really just indicators of what your actual credit score is with each credit bureau. To test it, make sure you pull your own score using the same bureau just before your lender pulls your credit score. You will likely find that the two are anywhere from 5 to 25 points different. This is intentional, as credit bureaus do not want you to be able to directly manipulate your actual credit scores through your actions. To get around this, they sell the consumer product, which will not match your actual credit score.
The credit bureaus have also announced a new VantageScore model. This puts all credit bureaus on the same page with an identical scoring model. In other words, they will now use the same calculation between them to develop the lender's product. A consumer product is also in the works, which will likely still be different than the actual VantageScore that lenders see.
Understand that whichever credit score you purchase, it will not be the same as what your lender will see. That can help or hurt you when it comes time for loan approval, and it can determine what interest rate you qualify for.