Mmmhmmm. But then, language is full of such things. (Brutalist architecture doesn't refer to anything being "brutal", either. It refers to "unfinished", where béton brut is architectural concrete that is left unfinished after being cast.)

The month-index offset you refer to is awkward, but does lead to one of the nerdiest jokes ever:
Why is Christmas like Halloween?
Because 25 dec equals 31 oct.