I always assumed it was something like this:

Pool A pays X per share. (i.e. a PPS system like deepbit)

Pool B pays Y=50/Z per share, where Z i the current number of shares  submitted for that block.

If you are mining for pool B, and get to the point where the total ammount of shares submitted for a block is so that Z>X, the miner would make more by switching his miners to pool A until pool B solves its block. Each additional share submitted to pool B until it starts a new block will be worth less than each share submitted to pool A.

Or am I completely mistaken?