Where is $expoprod defined, and what does it signify? I tried searching the code base, but I can't find the variable being set, or any useful comments.

In the scheduler.php and it has to do with production values on planets.

Let me see if I've got this right. As far as colonist increase on planets, it only modifies the growth rate if the scheduler runs more than one "tick" during an update pass?

It's a modifier use to calculate different values for different things on each planet.