There's also a difference between muscles and muscles.
One thing is long-term strength, meaning being able to carry weights etc. for quite a while. Over time, if you specialize in this, it gives you the slim, toned, muscular body type of a marathon runner or marathon cyclist. Their maximum strength is not great, but they have lots of endurance.
The other thing is short-term strength. That's what makes lift-weighters and wrestlers look the way they look, all bulky and bull-like. Bad endurance, but great maximum strength.
You can specialize in one of these extreme body types, or find a compromise if you want to have muscles. Karate guys and swimmers, e. g., tend to be in-between the extremes.
Therefore, I think it's useful to differentiate here.