The reality is no one knows how a muscle works or adapts, there are only theories, what we do know is that like everything everyone responds differently, and stimulating growth is no different.
I do think that working a group of muscles has a ripple effect (good and bad)on the rest of the body, it has to, the body is a system, as in it works as one.
As with all things you need to go by self evident truth, personally, if i just work the lower extremities, I know, at least my upper bodies condition is at least maintained.