Nearly everyone gets one surprisingly small monthly pay during his 1st year or so in a company.
In order to understand what happens, one has to know that workers earn their holidays this year for the next year. Once someone quits an employer, he gets a nifty paycheck which includes the monetization of the earned holidays that the employee did not take, yet. Now, once he decides to spend those holidays next year, he usually forgets that they have already been paid by the previous employer. That is, he actually takes an "unpaid" holiday from his current employer, instead of a paid one. Naturally, these days have already been paid before, but who cares, when the money is gone ;-)
All this is a prelude. Here is a nasty trick that makes the payslip look awfully unjustified.
The general rule is to adjust the monthly salary to take into account those "unpaid" holiday days not on a month-by-month basis, but once in a year, the month when the employee has taken more than 50% of the amount of the days off he was entitled to take.
Here's an example. You leave your old employer on December, 31. Next year, you take 5 days off during the first half of the year and then leave on vacation for 10 working days in August. Suppose this August had 20 working days. By the end of August, you will receive your salary for ~ 5 working days, as the salary calculation this month will take into account all the 15 "unpaid" days you have taken off so far.