I rarely have the same amount of air at the end of a dive. Most of my diving is as an instructor helper (gopher, flag dragger) so I come up when everyone else comes up.
Here's a hint...at the end of your next dive, hang out at the surface and reduce the amount of air in your tank to 500 pretty easily. I bet you can figure out how you can do that
If you are diving with classes often you should be well aware of how to do a proper weight check because it's a very common question/concern/problem with newer (and not so new) divers.