Dynamically update a graphic object in Mathematica

Question

I was trying to simulate a bouncing ball (with no gravity) in a unit box and I managed to come up with a solution:

pred[x_] := 1 - UnitStep[x] + UnitStep[x - 1];
DynamicModule[{pos = {0.1, 0.5}, vel = {0.05, 0}, dt = 1}, 
 Graphics[{{White, EdgeForm[Directive[Black, Thick]], 
Rectangle[{-0.2, -0.2}, {1.2, 1.2}]}, 
Circle[Dynamic[
 Refresh[(vel = (pred[pos]*2 {-1, -1}*vel + vel); 
   pos = pos + vel*dt), UpdateInterval -> 1]], 0.02]}]]

But there is one problem: the graphic object does not update as frequently as required (once per second), instead it seems to update about 10+ times per second. However, if I changed the motion of the ball to a simpler sinusoidal motion, the graphic object seems to update at a desired frequency:

DynamicModule[{}, 
 Graphics[{{White, EdgeForm[Directive[Black, Thick]], 
   Rectangle[{-0.2, -0.2}, {1.2, 1.2}]}, 
   Circle[Dynamic[Refresh[pos={0.5Sin[Clock[Infinity]]+0.5,0.5},UpdateInterval->2]],0.02]}]]

So my questions are

1) Why UpdateInterval in the first case does not work properly, and

2) Is there any other way to dynamically update the positions of some points in a graphic object if the trajectory of these points cannot be pre-calculated (e.g. with collision involved) or represented as a function of time?

Any help is greatly appreciated.