Different cannons shoot bullets at different speeds. Check out the (bonus) level 40 to just play around with a few different cannons at once, and see that their speeds stay constant whichever view you select.

I wanted to let the bullets have different speeds to change the 'strength' of the relativistic effects. For example, making the ones in level 9 move really, really fast so you couldn't pass through them without using a little relativistic trick.