HR module - "quantity" went crazy

For several days, the HR module was behaving correctly. We didn't change parameters, didn't add or remove employees etc. Only new 'leaves periods' were added. However, yesterday (Monday again), 'quantity' values for all employees 'went crazy': previously set actual leave limits were replaced with nonsensical values. Can anyone suggest what mechanism is responsible for making this mess?

So, it is not recommended to add leaves period the way:
Human resources/Leaves periods/[green +]?
Yes it is probable the reason of our problems - we ALWAYS do it such a way.

NOW I understand that correct operations are:
Leaves calendar-> To record or change your absences, double clicking on an existing date or absence
... as described in HR help.

Many thanks,
Zygmunt