The "unknown address" is also one of yours. Bitcoin-Qt pregenerates a set of 100 addresses (and only shows you the first one). Whenever you send a transaction, it takes an entire output (the term for some amount of incoming coin from a single transaction), sends whatever you specified to the recipient address and the rest to a change address, for which Bitcoin-Qt picks one of the pregenerated addresses.
This change address behavior can be quite confusing if you're new and it's not at all obvious what is going on without searching external sources for info (like this forum or the Bitcoin wiki).
Read more about it here:
https://en.bitcoin.it/wiki/ChangeYour wallet hides these messy technical details from you by generating the new address and then keeping it hidden from you. The address is still part of your wallet, and with some technical know how you can see the information in your wallet, but for the average user the wallet simply hides the address from you since it doesn't think you need to know it.