If you are leaving at the exact time each and every day, then that gives the impression that you are working to the clock and not working to the job. Obviously, your employer would prefer the latter. This impression is made stronger if you always turn up at the exact time each and every day.
If you are leaving exactly 8 hours (minus your 30 minute lunch) after you arrive, then the implication is that you spent some time arriving at your desk and preparing to work (starting your computer, grabbing a coffee) in the morning, and some more time in the afternoon shutting down and cleaning up your desk and things before leaving. This means it is perceived that you are not quite fulfilling your 7.5 hours work requirement.
Now, these impressions can be quite strong and generally are unaffected by the fact that you actually get your work done, or that nobody ever does 7.5 hours anyway because of toilet and coffee breaks.
The fact that you have a 1.5 hour commute and like to go the gym is almost completely irrelevant in your employer's and/or supervisor's view - everyone has to deal with a commute, and everyone has a life outside of work. In fact, some would argue that you should simply move closer to work (or get a job closer to home).
At the end of it all - you're not exactly doing anything wrong with what you're doing, and you're probably not going to be in trouble for it - but other workers who end up working 8 hours instead of 7.5 hours, and are less rigid in their timekeeping are going to be perceived as harder working, and more likely to be rewarded with raises, bonuses, and promotions, for the same output of work.