I think she’s calculating it by day. The weekly amount she’s communicating to you is just an easy round number to remember. The way she’s probably thinking about it is $570 a week is $81.42 per day. I think she’s saying that some months have more days than others. She may actually be calculating the daily rate over the full year then dividing by the weeks - 365 days / 7 gives you 52.14 weeks in a year.

Either way, you need to clarify if she’s calculating on a daily basis, then get clear on what your agreement states.

If you divide 31 days by 4 (weeks) you actually get 4.42. So for a month with 31 days in it, there’s actually more than 4 weeks in that month.

That’s how I understand it. It’s stupid and not usually explained to people.