another point is grip width.. now a lot of people do not believe in isolation of a muscle but i think even a basic ex like barbell curls can be manipulated. sure a muscle is trained as a whole but a small movement, body position and grip can change it. it shifts more stress to an area of a muscle group...take close grip bicep curls...the outer head.. wide...the inner area...
Of course! I never understood people who claim that variety is useless since any exercise that works a muscle works the whole muscle.
Take a look at an anatomy chart that shows skeletal muscle and you'll see that, especially with bigger muscles like chest and back, many of those striations travel in different directions. It stands to reason, then, that movements through different planes will emphasize certain parts of a muscle over others.
The "all-or-nothing" principle is too simplistic to use here.