Oh dung. The problem is in the Admin section Tools -> Store Manager.
Top button says:
Update All Products' Attribute Sort Orders
to match Option Value Default Sort Orders:
I tested by resetting a few attributes and then clicking this UPDATE button. Sho'nuff it reset those attributes again to 0.
I don't really remember clicking this at some point but probably did as I'm a new zenner and am learning the ropes by doing.
FAIR WARNING TO NOOBS LIKE ME: Don't click this button unless your attributes are properly set in the Option Value Manager (... just like the, er, text next to the button, um, clearly says... :).



