It think it only happens when your balance reaches over a certain amount. Say, XXX amount of millions (a lot? - I dunno - whats your balance?)
They round it down to the nearest million or hundred million or whatever, and put the excess into a high interest account maybe.
Then one day you might get the money back if your balance dwindles.
Basically your money is somewhere else, doing better things
Correct me if I'm wrong anyone!