It's more likely that the post processing is different, it's possible the 360 is using 10 bit HDR, it's impossible to tell.
X360 and PS3 do treat image output differently though, on a PS3 230 something is white and < 8 or so is black, this is a carry over from NTSC blacker than black whiter than white. On X360, 0 is black, 255 is white. So X360 has higher dynamic range FWIW.
Interestingly the reverse was true of Xbox and PS2, so it might be a result of the NVidia video out.
AFAIK On any HD signal there is no range clamp required, so X360 technically has the correct output, not that it matters. And AFAIK on any none transmision signal blacker than black an whiter than white aren't ever required.