Invoice Reminders

I have set up invoice reminders to go out to ALL clients. I have the following:

7 Days from overdue date

14 Days from overdue date

21 Days from overdue date

60 Days from overdue date

My understanding is that the reminders would (in my case) go out each week for 3 weeks from the overdue date. 

However, it looks like the reminders are going out on 7 days from due date and 28 days (21 after the 7 day reminder) etc. 

Would like to know if anyone else understands it this way.