Here's my school of thought... (and I'll be honest, I'd leave at least 1 player up as well). When defending a corner you don't care too much where the ball goes... as long as you get it clear. Accuracy is not the issue... clearing it is. When attacking a corner you not only need to win the header but need to get some degree of accuracy with it to be able to score. As such, if you've got a packed penalty box there is a lot less change of someone getting a free header compared with if there's less players there, where a bit of movement could result in a free header. I think this is also the reason why on a lot of our set-pieces we don't try and score with the first header, but look to win a header on the back post and it's that header that puts the ball in the danger area. That way, we don't need such accuracy on the first header for it to be a chance in front of goal. Probably total rubbish, but I think it's a logical reason!