This explains some odd behavior but it's not a big deal.
This is a monstrous fuckup of enormous proportions, if this is true then their "trading engine" is broken beyond repair (it wouldn't even deserve the name "trading engine" anymore) and should be taken down and replaced by something else immediately!
From Bitstamp: How does instant order work?
When you place an instant order, we add an order at the current bid or ask price (depending on the type of the order). If there is not enough bitcoins available at that price, your order will remain open until someone sells or buys additional bitcoins at that price. Our trading engine will then reposition your order every 10 minutes to the current order book offer.