I think if you leave early just because you feel your work is done and you "don't see a point in standing around" but then want to work "extra" hours to make up for it, that does sound like making your own hours. I know I took off early every week (to go to therapy) but had it all cleared with the supervisor, etc. so comparing with what others are doing is not necessarily the same as we don't know what their arrangements and relationship with their bosses/management are? I think you probably, when you want to leave early, especially because you have finished something, you should at least tell your supervisor you would like to leave and see if there isn't something else they have for you to do or if they agree that there's no point in your sitting around (and/or go over what work there will be later in the week, etc.)?