When any game goes multiplatform AT THE SAME TIME, it hardly becomes a good game! They need to spread themselves and allocate resources and not use the system's power.
Imagine this convo, Cell Programmer saids, "hey boss, u know this level here we can use the otehr SPU to help light up here to give a better effect, right now it a bit too dark when i spoke with the art direction last time."
Boss, "Nah, keep it the way it is, its easier to program for, we did this part of the stage on the xbox 360 already".
OR...
Xbox Programmer, "Hey boss, we can smooth out those edges on these grass here, and increase the frame rate on this jump slash move with anti-aliasing."
Boss, "Hmm, actually keep it the way it is, this stage is already complete on the PS3, when you change one little thing like that, frame rates may drop when we decdie to add more mobs later. Lets not bother, the gamers won't know the difference at all."
View