Late Registrations Coming through

We have a meeting set up with multiple dates.  Participants are registering once for multiple dates but we still have to approve them for each meeting.  The problem we are having is that once we approve a number of participants we go back in days later and see participants who registered a month or so ago that are JUST now coming through. 

This is a problem as we have deadlines on approving members and do not allow day of meeting approvals.  Is there a delay in the registrations coming through?  Why would someone who registered in October not show up until December?