13th salary in belgium


I have a question regarding the 13th salary in Belgium. I have just received mine from my employer with whom I started working since November 2016. I can see in the payslip that the gross amount is the same as I get every month (4.300) but my net amount is much less (1.700) compared to my normal monthly net which is 3.200. Does anybody know why there is this difference? I had the impression that  the 13th salary is a full extra salary.

Thank you for your answers

Your 12 months has tax calculated with 1/12 th tax free allowance, you4 13th month has no tax free amount and your salary is so high that tax and social security is around together 60%.

Thank you for your answer