How it works is if you were to send 19.8mBTC to someone and you had 0.5BTC in your Bitcoin address, your client will actually spend all of the 0.5BTC and send back the remaining 0.49~ BTC change to you at a new Bitcoin address, which makes it harder to track but also confuses lots of new users as they expect it to go back to the same address they sent from.
Blockchain.info is different from most wallets in that it sends the change back to the
original address.
Also, in case it isn't apparent, 1 mBTC = 1/1000 BTC and 1 bit = 1/1000000 BTC.