Two reasons.
One, the data that each agency has could be slightly different. An account missing here, an account present but data different there, etc.
Two, the different agencies are known to be slightly more or less sensitive to the same data. For example, a TU score generated with a profile where 40% of the revolvers reporting a balance may not see the same scoring ding that would be seen from an EQ score, where over 33% on EQ is said to give a ding where that number is believed to be over 50% on TU. This is just one minor example where there are probably dozens of possibilities.
