So, I see this is from more than a year ago, but I've been having this problem lately as well. In one particular household--they live in the Pier Palace house in Windenburg--periodically Sims will take a meal from the fridge, walk past the dining room table (with high-end, comfy chairs) and sit on the bed in the downstairs bedroom to eat. I've seen them do the same with a beverage. There is never anyone else in the bedroom when they do this, but often there is someone working on the computer in the office next to the bedroom.
So if they're choosing to sit near other Sims, they must be reading that the bed is the closest seat to another Sim, in spite of the fact that the other Sim in question is in the next room! It doesn't happen every time, but it happens often enough to be annoying, especially when they leave their dirty plates on younger sister's dresser.