This is because of the TM local snapshots. Go to TM, click options, and remove the "create local snapshots" option. Your missing space will be back within 5 mins.
Interesting indeed, that it's backing up my system even though Time Machine is set to off. However, how smart is this local backup, will it release some space if I decide to download something big when the space is running low? Or will it just keep going until I have nothing left?