According to MyFICO, the three credit bureaus that determine an individual's FICO score, Equifax, Experian and TransUnion, are equally accurate. They are also typically similar, as the same mathematical equation is used by all three agencies in calculating creditworthiness.
Since different lenders report their information to each of the three bureaus at different times, one bureau may have more up-to-date information than the others, resulting in different scores across each. Therefore, the scores are most similar when accessed at the same time. However, a score pulled from TransUnion today could be significantly different than one retrieved from Experian one month ago, especially if recent payments or delinquencies have been reported.