Originally Posted by Keyser Soze

The balance can grow a little now and then when pending charges are dropped. Some places put a $1-$5 pending charge on your account when you signup to make sure the card is legit and 30 days after its dropped and added back to your account.

The balance can also drop a little especially if you have made transactions in non US$. When you use the card the merchant use the exchange rate that day to make the pending charge and when the transaction settle a day or two later the exchange rate can have changed a little bit.