In the *.h file for the Xmega32A4 the last vectored is #define'd as #93 . For the 'A4U, its *.h file has the same vectors up to that number also, then its last two vectors are #125 and #126 ( USB related ) . Why wouldn't they use numbers of #94 - #95 ?
Edit: Maybe it's to have room future devices... ? I just checked the '256A3 and it has a jump too from #93 to #104 ( so all the *.h files probably have the jump ) .