why would someone mute on messenger but not block.

Have you ever PM this person before? Believe it or not many people don't know there is a PM feature on FB

yes, we've talked online before, and hung out. My guess is she's busy with schoolwork. I guess I'm just curious, but I guess it doesn't really matter anyway.

Because they don't want to be interrupted by notifications but still might want to peruse the messages at another time.