Pension contributions in Spain

Hello,
I have received an offer for a job in Barcelona. I currently work in the Netherlands. My future employer would offer 2% contribution for my pension and these contribution is on a voluntary basis up to 5%. How can this be possible?

I read in many forums that pension contribution is 23,6% in Spain, of which only 4,7% is paid by the employee.

Thanks in advance if anyone has an answer on this!

Valerio