Ok, first off, the reason you are seeing activity on one core, is that your game is synchronously threaded (run as a single sequential stream of instructions), so it can't make use of the multiple cores.
Second the reason you are seeing such a similar temperature, is because the cores are so close to each other, and because you are not really generating very much heat at 49-51C. Since the cores are close to each other, and no one of them is substantially hotter than the others, the ambient temperature of the whole CPU is at 49-51C. This is due to the second law of thermodynamics if you ever want to look it up. CPU cooling affects the whole chip, not specific cores, so the whole chip will be cooled uniformly. as Cooler components will not give up heat as readily to active cooling as hotter components, and hotter components will dissipate their heat into the cooler components, and the cores are almost as close to each other than they are to the heat-sink apparatus. As heat is pulled out of the CPU, the hot core is feeding heat into the cool cores at about the same rate.
I posit that if your one core was approaching 60C, you would start to see a more marked difference in core temperature, but you will probably never see a core more than 10C different from the hottest one, just due to their proximity, and this is actually kind of good, since wide variations in temperature between components would significantly contribute to their wear and breakage. tiny components heating and cooling rapidly and inconsistently tend to rip the connections between them apart.