It is possible there is another factor involved in mountains that isn't apparent. The combat code is quite complex.
In my current game I just did this with 7 infantry and one armored car of mine against a single infantry in core mountains and did not lose any units of my own.
since it does it 12 out of 12 times I would almost call that expected behavior
There are many things about this game that are not apparent from "book learning" but you learn by experience. This may be one of them.