I've noticed the people in this house have a tendency to never finish off a container of anything that's in the fridge.
If there's like a glass worth of milk left in a carton, and a new one arrives, it is immediately ignored in favor of the new carton. I don't drink milk that often anymore, but it seems I'm usually the one to have to drink it and throw it out, and by then half of the other carton will have been drank through.
Also today I threw away 2 cartons of eggs that I had been noticing in there for a while, upon checking them each only had 1 egg inside. MORE OF THIS SAME PROBLEM. They hadn't expired yet, but I threw them away OUT OF SPITE.
Same goes for mayo if there's still a small but usable amount left in the bottle, and ketchup, and mustard, etc etc etc
Maybe they're pack rats!