Dynamics calculations are computed in a very complex way.
Many tasks start when dynamics is needed and it's impossible to know which one will terminate first.
So the results will never be outputted in the same order when you recalculate the animation.
Furthermore many calculations are interdependent and the random completion will actually put a small amount of randomness in the final result.
This is a small bit of noise, but it will be added for every frame, so the longer the animation and the more different could be the animation if recalculated.
To this you have to add the fact that every object in Maya is made of triangles, even nurbs, so a cg sphere will behave different from a real sphere and so on...

Caching has been created also for this problem (not only for speed reason). So if you should create a cache for an animation with dynamics and keep it if you are satisfied or delete and recreate it if you don't like what happened.

Bye!!