Half of the seats of the Bundestag are decided winner-takes-all in the districts according to the "first vote" (Erststimme). The other half is divided proportionally over the party lists according to the "second vote" (Zweitstimme). If a party wins a greater share of the districts (Direktmandate "direct mandates") than of the proportional vote, the number of seats in the Bundestags increases by those Ueberhangsmandate. There is of course a fixed number of districts, so you would expect that the number of seats in the Bundestag is that number times two. But it so happens that that's not the way they do it. Don't ask me why...

If you can't convince them, confuse them. (Harry S. Truman)