a - b isn’t just a.__sub__(b). It also tries b.__rsub__(a) if a can’t handle the operation, and in the 1 - 2. case, it’s the float’s __rsub__ that handles the operation.
>>> (2.).__rsub__(1)
-1.0
You ran a.__rsub__(2.), but that’s the wrong __rsub__. You need the right-side operand’s __rsub__, not the left-side operand.
There is no implicit type conversion built into the subtraction operator. float.__rsub__ has to handle ints manually. If you want type conversion in your own operator implementations, you’ll have to handle that manually too.