I found the reason. If the price is 100 the vat (20%) will be 20 and the sum will be 120.
But if the price is 150,50 the vat will be 30,10 and the there is an error because toNumber cant handle the comma. I need it to calculate so that everything ,49 and below are rounded down and everything from 50 and upp is rounded up.
Hope that I can get help with this.