You go check in, and if there are two people traveling together, you'll get a window/middle or middle/center. They'll fill the other window or aisle with a single traveler. Unless you check in last with nothing left.

They'll fill it up that way even if the flight isn't totally full, in my experience. So, you may see all 3 seats filled up near the front, and then a few very empty rows in the back.

I can totally believe moondog.