Payout from slush's pool

[apis]

I've been mining on slush's pool for a while and figured I should try another pool to compare with (and try to support one of the smaller pools as well). The only way to transfer the bitcoins I've found is by setting the payout threshold to lower than the confirmed amount. The lowest value alowed is 0.01 though and when doing so the pool leaves ~0.01 btc on the pool. Is it impossible to transfer that last 0.01 from the pool? And if so, why?