It looks like the roster is something like 6-7 days on/4 days off. Always an early start on day one and a late finish before days off.
You have 12 days off
pr month - unless you take annual leave, then they deduct 0,4 days off
pr annual leave day (and round it up), so with one weeks annual leave you will end up with only 9 days off - and a roster that looks like 6-7 days on/2-3 days off.