It's down to your toolset as he pointed out. If the game toolset/engine is heavily optimized for PC it will go okay to PC but struggle with PS3 without additional overhead - this was the problem with the Orange Box, for example. The Source engine (no relation I'm sure) was built for PC and moved easily enough to 360 but struggled on PS3.
Unreal games are the same although I think recently this has improved a lot. But the engine was initially built for PC only which gave PS3 development issues.
Here we have generic tools on PC designed to support multiplatform - a big difference. If ID are correct then they too have a good set of true multiplatform tools.
While they're not identical the PC/360 are close enough to allow easy ports/toolsets. The PS3 is different enough you either need to put in extra effort, use true multi-platform tools or develop specifically for it.
At least... that's what I read on the internet. So it must be true.
Try to be reasonable... its easier than you think...
