German voters get two votes, one for their candidate of choice, and one for the party of choice.

A party gets as many seats in parliaments as is proportional to its share of that second vote. That is, if a party receives 40 % of those second votes, it gets 40 % of the seats in parliament.
If more of this party’s candidates get elected directly to parliament via the first vote, than it would get via its share of the seond votes, this party receives additional mandates, the so-called overhang mandates.

Now, for a small party like the Free Democrats it is highly unusual to ever have a candidiate elected directly to parliament. It’s only way to get seats in the parliament are the second votes. The voters of the CDU know this, so if it looks doubtful that the Free Democrats, as the party they want to form a coalition with the CDU, will get enough of those second votes to make the 5 % hurdle, many will vote tactically, that is, give their first vote to CDU candidates, and give their second one to the Free Democratic party.

That explains why many voters who usually would have voted for the CDU gave their vote to the Free Democrats. Besides their weak election campaign, that is the reason why the CDU got less, and the Free Democrats more than expected.

I should add that until two days before the elections, when the last polls were taken, up to 30 % of all voters hadn’t made their mind up yet whom to vote for, or even if they would vote at all. So with this high margin of uncertainty, regular CDU voters had a better reason to vote tactically for the Fre Democrats than usual.

If you want to know some more details about our election system you can find a primer here

I’d be interested to hear your thoughts on why the Free Democrats did better than expectated while the CDU seemed to perform so disappointingly…