That's just the way Bitcoin works... It isn't like a normal bank account, where you receive a bunch of payments and then when you go to pay someone, it just takes out the exact amount you require. With Bitcoin, any input you get, has to be spent in FULL when it is used.
As an example, let's say you've received a bunch of Bitcoins transactions worth $100, your total balance says $100, but it is actually made up of a combination of all the transactions you have received:
ie, say you had 5 incoming payments:
What you actually have, is 5 chunks of money... rather than one big pile of $100. Now, as mentioned previously, the thing with Bitcoin is, that when you attempt to use one of these chunks,
you have to use the whole thing.
So if I want to send say $37 to someone... I can't just send out $37... as I have no $37 chunk. Instead, several chunks get combined until we have at least the required amount. ie. the $25+$30 chunks (as wallets tend to use the oldest ones first). These get combined and then broken into 2 new chunks as required. So a $37 chunk would go to the person, and the remaining $13 chunk would then be returned to a change address in my wallet.
so... it would be: Input1 ($25) + Input2 ($30) => Output1 ($37 to the person I'm sending to) and Output2 ($13, into my change address - this is the "extra" money you're seeing being returned to your account).
Hope that isn't too confusing... it certainly caught me out the first time I tried sending bitcoins... I freaked out when I first tried to send $1 to someone... and it used a $10 chunk!
Then I saw that I had $9 returned...
