I see where your coming from but i dont completly agree. But i agree there are several types of rolling which based on what i said could be classed as drilling.
Rolling where i try work of my back.
Rolling where i try get on top
Domination rolling (Awaiting porn jokes lol)
few others, So i see what your trying to say its just when im rolling my aim is tap my opponent but when im drilling my aim is to do whatever im working on. Might be a sub/pass or whatever.
Hope i made sense