Credit scores vary by the type of credit for which one applies. A credit score for a mortgage loan will be different than an auto loan, a credit card, insurance, or employment. There are at least a dozen different scoring models provided by Fair-Isaac and the major credit bureaus, plus many creditors use their own scoring models.
Mortgage lenders must use a special type of credit report, commonly known as a "tri-merge" that combines data from all three of the major credit reportin agencies. Since the tri-merge reports are compiled by thrid parties, it is common for credit scores from each bureau to vary slightly from compiler to compiler. Very rarely will the score you obtain from a consumer service (such asTruecredit) match the score obtained by a mortgage lender.
The reason for the variance in scoring and various scoring alorithims is that the purpose of a credit score is to predict risk of granting new credit. It is not a point system like football or a grade like a school test.
As Christopher correctly states, in most cases the scores won't vary much from credit product to credit product. However, it is not uncommon to see a wide variance from bureau to bureau.
Prior to entering mortgage lending, I spent 12 years in the advertising field developing prospecting and credit decisioning programs for several banks and captive auto finance companies. I've purchased millions of credit derived records for credit offers, worked with Fair-Isaac to develop proprietary credit scoring algorithims for my clients, and reviewed in excess of ten thousand credit reports. The amount of misinformation and outright myths about credit scores, scredit scoring, and "how to beat the system" is jsut amazing.
Christopher, Experian stopped reporting its own data to Fair-Isaac becuase they now compete directly in providing credit monitoring systems to consumers. Experian decided to stop "arming" the competition with data, so to speak. However, that decision does not affect the relationship between Fair-Isaac and Experian for purposes of credit decisioning for lenders.