I am probably missing some background knowledge, but the salary calculation process in Belgium looks like it has been subverted by Ministry of Finance officials.
This is better seen by example
The so-called key formula for the tax prepayment calculation (précompte professionnel) for the year 2008 (income year 2007) states that the first 5220 EUR are exempt from taxes.
Now, let us work by example. We will do the calculation for someone called Jacques who earned 10000 EUR in 2007. We proceed by subtracting 5220 EUR from his earning and we pay taxes on the rest, that is, the 4780 EUR.
This amount falls entirely within the lowest tax range, that has the tax rate of 26,75 %.
4780*0.2675 = 1278.65
So Jacques supposedly has to pay 1278.65 in taxes. Right? Wrong!
The calculation procedure is defined by the Ministry of Finance as follows
First, Jacques calculates his taxes on the total of his earnings. 10000 EUR are split in two ranges. The first 7550 EUR is taxed at the rate of 26,75%. Everything over 7550 EUR up to 10250 EUR of earnings is taxed at the rate of 32,1%.
7550*0.2675 + 2450*0.3210 = 2806.08
Second, Jacques subtracts from the result the amount that he would have paid if the first 5220 EUR were taxable at the lowest tax rate, that is
2806.08 - (5220*0.2675) = 1409.73
The difference between both calculations is
1409.73 - 1278.65 = 131.08
That is, 131.08 EUR earned by Jacques have fallen in the deep pockets of the state solely because of a particular interpretation of the law that the Ministry of Finance has made. I have no other explanation for this phenomenon.