It's because there are frequent negotiations. It's not that you're changing governments every six months, but there are more frequent negotiations.
The result is the following. You can have a change in government in essentially in three ways. You can have an election that changes the composition of Parliament under government reforms, or you can change a leader, the head of a coalition, or you can change the parties that comprise a coalition. Especially that third type of government change is one in which you inevitably get more distortion between what voters voted for and the policies that result, because you end up having policy bargains that weren't in the discussion in an election, but instead are the backroom negotiations among parties to try to cobble together another government.
All I'm saying is—