rough explanation:
When you send a transaction in bitcoin-qt it will use a higher amount of Bitcoins and transfer some of it to a new address.
It's kinda like change, you pay $5 dollar with a $20 banknote and get $15 dollar back.
Because this address is missing on blockchain.info you will not see the balance there.
Btw great addon for bitcoin-qt to manage your addresses, it allows you to see the change addresses:
https://bitcointalk.org/index.php?topic=144331.0I think I understand. But then which is accurate? Block or qt?