It's the Fezzik rule. You learn different techniques for fighting many people, than you do fighting one person at a time. So it seems to be with monsters. There's a design technique when they are supposed to fight many people, and a different one when they are only supposed to fight one at a time. A consistent source of irritation for me is when the big bad monster that I fear is going to TPK the party goes down in two rounds because of the heavy co-ordinated assault that the PCs lay on it. Hopefully by designing some monsters to take on multiple PCs this won't happen as much, and by coming up with a naming scheme this makes it easy to pick up of the books when you are designing an encounter.