Michael Brundage who is working on the backwards compatibility has a small webpage talking about the challenge of making it work here:
http://www.qbrundage.com/michaelb/pu...s/xbox360.html
It looks like the reason they're using different emulators is to get better performance out of the games. I'm sure there are 1-2 emulators out there that run Halo and Ninja Gaiden perfectly but doesn't run Tony Hawk's pro skater right or vise-versa... I dunno..