Day-to-day work is necessarily collaborative and productive... Whereas when at the board in an interview one person knows the "answer" and is across the table testing the other. It's a completely different set of pressures and requires different skill.
Further, I must have one of those faces people pity because its often that when I end up at the whiteboard the interviewer wants to "help" me if I don't spout off the answer immediately (I've learned to at least start talking so they will give me a second to think). This sometimes entails them trying to lead me down a path which is not the way I would have approached the problem. I'm confused, they think I'm an idiot, now I'm flustered, could this interview go any worse?
I've learned to politely ask for a second to consider and usually that completely solves the problem. I've been successful in technical interviews -- but the idea that it's the same thing as when working out hard problems with team members seems a little silly?