[JRTJH]

The "answer to the age old mystery" is really quite simple. If you look at "older water heaters with the AC switch, you'll see a tiny "safety pin" hole in the switch rocker. That hole is under the "ON" side of the switch. The purpose is to
prevent the switch from being pushed to the "ON" position when the safety pin is installed.... Given the switch location, it would be impossible to insert the pin if the switch were located with the "ON" part on the upper half of the switch....

Suburban apparently didn't want to "change switch positions" (where OFF/ON are located) so they left the switch functions as they were previously even though the safety pin hole has been discontinued.