iStat reading different from Activity Monitor

I would trust Activity Monitor. I want to say that iStat there is a delay from what is actually correct.

As for school Mac vs. your Mac. Remember that your school's Mac's are setup specifically by IT so it's most likely that it's only running things that are essential where as your computer is free to do whatever you want with it so you might have background processes or whatever running in the background and depending on computer, if you have a portable with a integrated GPU that is also taking up your RAM/memory. Your Mac's at school are probably Mac Pro's or Power Mac's that all have discrete graphics that have their own memory.