Late Charges

Hello - Has anyone used late charges?  I am unclear on how the process determines which invoices to apply late charges to.  The customer I am testing with has open invoices from current to 400+ days late.  The late charge type is adjustment, the charge calculation is overdue trx only, days in period is 30, receipt grace days is 75 and the charge beginning date is 01-jul-2018. I thought it would choose all trx over 75 days old.  However, it chose a handful over 75 days with due dates in July.  Does the days in period limit the selection window?