I'm not sure if it was because i got bitcoin in the timeframe or there is just a bug in the GUI of the addressbook window.
Anyone else experienced this bug, or maybe this is a "normal" behaviour which i haven't found in the docs yet?
It is expected behavior, though probably those addresses should be labeled more descriptively. Bitcoin automatically generates one of these no-label addresses whenever you receive BTC at the address listed in the "your Bitcoin address" area. The new address replaces the old address in "your Bitcoin address" to encourage you not to re-use addresses.