Common names are just common names and there is often no obvious rhyme or reason why any particular common name developed :-) One possibility is that in comparison to the other US native hydrangea, H. quercifolia, arborescens does have smooth leaves.........

The arborescens leaves do look thinner than those of macs and oakleafs. The oakleaf leaves have a rougher look, specially those leaves that refuse to brown out, dry out and fall down. The oakleaf leaves now remind me of the look of very old leather in cars, when the leather cracks up a little due to sun exposure/etc.