This nails it. The nominal sizes are mostly (for some insane reason) based on the outside diameter of the wheel and tire. 27" road tires would have a 30mm tire size, whereas 27.5" mountain bike tires are probably running close to 60mm these days (haven't been keeping track of what's in vogue).

ETRTO is based on the bead seat diameter, which is what matters when you're buying a tire.