Holidays/Annual Leave (Riyadh)

Can anyone explain why my employer is counting my days off as part of my annual leave? I feel I'm getting completely cheated.

I am suppose to get 2.5 working days for every month worked, a total of 30 days a year. Key word here is *working days*

I do shift work in a hospital and my normal month is 16 shifts..

I just took 15 days annual leave there with 4 accrued holidays but I'm still doing 6 full 12 hour days in that month.

If you include my 15 days annual leave (that are suppose to be paid) plus the 6 days I also worked on my month rota I've done 21 days.

My working month is suppose to be 16 duty a month so I'm working 5 extra shifts for free?

What is going on?