If you really are studying with perfect productivity, then you need to ask yourself if you are triaging your time properly. For instance, say you have three tests: one easy, one medium, and one hard. Do you study equal amounts for all of them? Probably not. If you have already spent seven hours studying for the hard test, how much will one more hour help you? Maybe an 85 to and 87? But then if you spent that hour on the medium subject maybe your grade would jump up from an 80 to a 90. Thus your time would better be allocated to the medium test.
There is an economic concept about this though I cannot remember its name. Marginal Returns and the Law of Diminishing Returns, I think.