How apes with a similar muscle mass are so much stronger than human?

There's a few reasons but functionally most of them come down to finer muscle control. If you look, for example, at muscle bundle innervation we control much finer sets of muscles at a time. Great apes, on the other hand, fire larger bundles at once resulting in stringer but jerker motions. Mechanically speaking there are ask changes in insertion points to control leverage advantages vs mobility advantages as needed. Just watch a chimp doing things that require fine control and they look very clumsy compared to a human. Also, keep in mind that there's learned flexibility in both systems and humans can be quite strong and some chimps have learned to be quite dextrous (I.e. The differences of the distributions are further exaggerated by the tendencies of the species). Basically though, you're seeing the result of 6 million years of evolution in support of shorter reproductive cycles & increased intelligence and took use, the musculature and nervous system changes were critical to support those advancements.