Payroll is expressed as a cost per MONTH. However, you pay it based upon a cost per DAY. Therefore, the expense you have each day is calculated as follows:
Daily Payroll Expense = Monthly Payroll Expense / Days in current month
Then, you can use that to arrive at your weekly expense:
Weekly Payroll Expense = Daily Payroll Expense * 7
If you want to play with the math, you will see that holding your monthly payroll constant and varying the number of days in a month will result in a lumpy weekly payout like you are seeing. February tends to be the most obvious month for this.
Note that the same thing happens for leases.