Scores differ among CRA's when your reports have different data. This can happen when an account isn't reported to all three CRA's. Sometimes delininquencies aren't reported to all CRA's.
The most common source of score differences arises because inquiries to your file when you apply for credit are rarely distributed evenly across all CRA's. If a greater number are pulled from one particularly CRA, that CRA score will be depressed more by inquries than the others.
Each CRA has historical roots within specific sections of the country and when a creditor pulls a report for an individual in a given section, they tend to pull a report from the corresponding CRA. Consequently, it's often the case that your score is lowest for the agency that has the strongest relationship with your area of the country (if you've actively been seeking credit).
- Harry