Pie chart not displaying correctly

Hello everyone,

My table is as follows:

however, when I create a pie chart with headcount and department, the headcount is more than the actual headcount for the month. 

For instance, for Janaury 2023, my headcount is 183 but when you add all the values of the different departments we are at 204 headcount; meaning I have 21 employees more. 

I suspect the problem to be those who changed department at some point of time. In my table above, employee 1189 changed department from 6 to 7 in March 2022. Would that mean he is appearing in both department 6 and 7? If yes, how can I resolve the problem please?

Having the same issue on Age, employment type,  category.. 

Thank you so much for your help

Ariane