well I think this much of a difference in distance is caused by the inaccuracy of the simulation at a lower framerate (just like the case where the jump arc can turn out a bit lower when games run at very low fps)

Why it starts moving left I don't know, it's probably continuously bouncing. Might also be related to inaccuracies (standard lite-c vectors only use var).

You could try switching to float vectors or calculate the bounce vector yourself, not sure if thaat helps, though.