Us according to Finn

So I know that kids come with their own little personalities and sense of self, but I really thought we'd get to choose what Finn calls us (at least until teenage days, right?) We have always addressed ourselves as mama and papa, saying these words deliberately and frequently. And no, I wasn't hurt when Finn's first word was ball, then bird, then book, then papa, but then .... mom? Come on son, does it really have to be mom? Not that there's anything wrong with mom--it's what I call my own mom, after all--it's just not what I wanted, and not what he's ever heard us say. He also seems to be leaning towards dada for Matt, again, with absolutely no prompting from us whatsoever. We literally have had the following exchanges multiple times lately: 

me: mama

Finn: mom

me: mama

Finn: mom 

and 

Matt: papa

Finn: dada

Matt: papa

Finn: dada 

 What's up with this?  Could a kid really come programmed this way?  I guess it's time to start letting go of any plans we have for him, even the most innocuous ...