Actually, that doesn't surprise me. The OS needs to specifically support AltiVec (in particular VRSAVE on context switches), and different G4s have different demands. For example, while 9.2.2 uses a very generic AltiVec OS support routine and works with just about any G4, firmware permitting, 7447s won't work in OS X prior to 10.3.5 or so.

The G3, on the other hand, is just a tuned 603 plus/minus backside cache.