[pixelbill]

I think you nailed it, you are lucky.

Most jobs do not function like this, to think otherwise is delusional. At every single employer I've ever worked for, if you left early or came in late despite accomplishing all your work, you would get in trouble. I have seen it happen over and over again.

Typically, they will claim that you could have accomplished more work if you were only there for the extra time, regardless of the reality of the situation. Your time outside of work is worthless to the company, and they would rather waste your time so that they can (possibly) squeeze another drop from you.