I used to work at a company where we could clock in and out if we wanted. If we did, we could keep any extra hours at our convenience as a day off. If we didn't it would be a full workday by default.

I liked the freedom to work long days every now and then, and then take some time off later when I wanted a long weekend or so. A colleague decided that's a great idea and started doing the same, but stopped after a week or two when he realized he's just accruing negative time.

I'm not claiming that I was fully productive with my office hours, just agreeing with you that some people really do think they work more than they actually do.