In short, it is also for the same reason that you can have one guy with 600 levels of natural test compared to someone on TRT with 600 levels of test and the guy on TRT will be able to build muscle easier and shed fat easier- androgen sensitivity. Exogenous test increases this and in some cases, if you are genetically prone to MPB, it can speed up the process. Depending on the method of delivery (patch, gel, subq vs IM injection) there can be variances in levels of test, DHT, e2, etc.. IM tends to see the biggest swings, while other methods are more linear.