Is it a postdoc financed by the EU? Then it would make sense, as they need to pay employer contributions from the gross salary. If it is Swiss, like SNF funded, I am not sure if it is normal.

That said, it is actually a good salary for a postdoc in Europe, and even after tax you will earn a lot more than a lecturer in the UK in many instances (30 to 43k in pounds *before tax!* This is for a job I know someone is applying for).