Differences in the ECU number. As was explained to me today while I was having mine done...

There are different ECU numbers based on when the car was manufactured. As they manufacture new models, they may make adjustments to existing settings and will therefore change the ECU number to coincide with those changes.

Upsolute have programmed a few Crossfire chips and have programs already on file that may match your ECU number. If not, then yours needs to be read and a program developed.

Upsolute ECU Upgrade