Probably just the way they're marketing them, although I note that Starrett itself has a video on its website explaining the use of combination squares in woodworking.
Several years ago, I found a Starrett combo square rule at an estate sale marked, both sides, in eighths and sixteenths. It was too far gone to buy, but I sure wish they'd offer such a thing again, made to machinist standards. I can't read the fine marks anymore.