Most of the credit scores you buy or get free online are not the exact ones used by lenders. The gold standard is what's called a FICO score, named after the San Rafael, Calif.-based Fair Isaac Corp., which devised a mathematical model to predict the credit risk of consumers based on information in their credit report. FICO is the model most widely used by lenders.

"The confusion and irritation of (consumers) is understandable," said Craig Watts, public affairs manager for Fair Isaac. "It could be avoided if Experian, TransUnion and their various consumer Web sites would only be upfront to consumers about the nature of the consumer scores they sell — they aren't FICO scores, they aren't widely used (if used at all) by lenders. They are at best someone's estimate of what the consumer's true FICO score is."

To be fair, Experian and TransUnion do have disclaimers that inform consumers that the scores they provide aren't FICO scores. But the information is buried in fine print that many people probably don't notice. When you order your credit score from the three bureaus, here's what you're getting:

Equifax's credit score is called "Score Power." Even though this uses the FICO scoring mode, you may get a different score than one pulled by a lender because the information in your credit file is constantly changing, which means your score can change as well. The score you get this week may not be the same score a lender would get from the credit reporting company the next week.

Experian calls its credit score product "PLUS Score" and is based on factors similar to but not the same as FICO. The PLUS Score ranges from 330 to 830.

TransUnion's credit score product is also not a FICO score, but based on the bureau's own proprietary scoring model, which is why the Los Angeles reader's score was outside the FICO range.