The problem is likely that in Bitcoin-Qt, your balance is not kept in a single address. For privacy reasons (of the system as a whole), change from transactions is sent to new addresses.

The abstraction provided by the Bitcoin-Qt client is that of a wallet, in which coins enter via (don't reside at) addresses, and are sent out from it, and is otherwise a black box. When you look at it via blockchain.info or similar sites, you get a more low-level via, where "balances" are kept per address (which is actually sort-of misleading, as at the protocol level, Bitcoin doesn't work in function of addresses or balances at all, but in terms of transactions on individual coins).

This is normal. This has to do with the way transactions work in Bitcoin.

The amount of satoshis in the inputs and outputs of a transaction must match. (In reality the inputs can be higher than the outputs, the remainder then becomes the the transaction fees.)

Transaction inputs are based on previous transaction outputs that were sent to public addresses you control. If you send an amount that is not exactly what you received with a previous transaction, the software creates a transaction with a larger input and sends back the difference to a public address you control.

For privacy reasons, the BitcoinQt software creates a new address (instead of re-using the main address), and sends money to it. There is a pre-allocated key pool of such addresses. The default size is of 100. The key pool grows as you use them.

For more details, see this blog post: http://blog.barthe.ph/2014/04/03/bitcoin-balance-differs/