There are a number of reasons why:
1. The school arrival date of the staff member i.e. if the staff member's school arrival date is after the start of the academic year, then the possible sessions attended will be different to those with arrival dates before that.
2. Not counted absence codes that have been assigned to a staff i.e. Late (L) or Part Time (P).
3. There are missing attendance codes for the staff member.
All these reasons will effect the total possible sessions for a staff member.