The vacation leave assessment for biweekly employees occurs every other pay cycle because for biweekly employees their leave accrual eligibility is based on a four-week cycle (two biweekly pay cycles). In order to perform the assessment the payroll system must reference both a B1 and B2 pay cycle this means that sometimes it must refer to a prior months pay cycle in order to know the full gross pay on which to base the assessment. This occurs anytime the B1 and B2 do not fall into the same month.
To predict when this will occur refer to the Payroll Processing Schedule. Anytime you see that the first BW in a month is a B2, expect that the payroll system will need to reference a prior months pay cycle in order to calculate the leave assessment.