Think of it as Chopping versus Carving.

If you have a cleaver and chop a piece meat, you have to swing (more arm motion)

If you want to carve a piece of meat to specification, you need to use smaller, more precise motions

They differ because what you're trying to do is different. Using the above example, it's the same piece of meat but it is being handled in different ways and for different reasons.

The same for guitar; you are strumming because you are playing a broader rhythm which takes more motion.You would pick single notes in a solo using less motion and different muscle groups because you are trying to hit smaller, more precise notes.