This game utilitzes the Hedgehog Engine, which as an aside I'd like to state is just a rather powerful global lighting engine, not a physics engine. That one's done by Havok (or PhysX on the Wii.) The only other game that used it was Sonic Unleashed. Problem is with all the detail Sega puts in surrounding environments, on levels, on objects, on things exploding everywhere as you zoom right by them, it gets to be somewhat taxing on the system.
For that reason, the Xbox 360 version had a limiter on its framerate. The game was capped to run at 30FPS, and you'll notice on some of the more intense moments that the framerate still drops (especially in places like Adabat, the Southeast Asia stage.)
The PlayStation 3 version, for whatever reason, doesn't have this limiter on. The game runs at 60FPS, but jumps around at times to 45, sometimes to 30, and infrequently to levels lower than that during more infrequent scenes.
Sonic 2006 runs at 60FPS (usually), but it also doesn't have as much going on and doesn't have to load global lighting effects from the Hedgehog Engine.
As for Sonic Colors, that one I'm not sure about. We know the Wii is quite capable of handling 60FPS without much of a problem, but perhaps Sega put it in as a precaution? It doesn't use the Hedgehog Engine (as they claim the Wii isn't strong enough for it) so I can't really think as to the reason so much as a few levels have a lot going on that 60 to 45 dips would be occasional.