The thing about Axl is he is a chameleon model. Though he can't change into ANYTHING, he can shift into things his own size. This is what makes him such a valuable member of the team. As shown in Prank 7, it also has *ahem* other uses. ;)

That, and the surprise expressed by the two of them is because X is the peacekeeper of the group. For him to do that is just unheard of. =P