Erm.
The framerate of any game from any system is dynamic unless it's *capped* at a certain value.
I can tell you right now with 100% certainty that the frame rate of Motorstorm and MLB 2k7 never gets near 60fps. I'd estimate that an average value for Motorstorm in-game play is around 30fps with dips below 20 when the screen is full of action.
The number of frames rendered per second is totally dependent on what the system is rendering at that exact moment. It's exactly the same as a PC game. If you turn the details up for a given game, the minimum, maximum, and average FPS go down accordingly depending on the horsepower of your system.