I’d guess that nomenclature originates in the world where every small US Main Street had a stationary store carrying all manner of paper sizes and stocks for diverse purposes—none of which involved use in anything more sophisticated than a typewriter.

One particular “standard” that sticks out in my memory was “math paper”, which I recall as being unbleached, about 5” x 8”, and used pervasively in primary education (at least in New England) into the 1990’s.