To my knowledge, passengers who haven't pre-selected a seat or checked-in online have a seat assigned to them by a computer algorithm that runs several hours before departure. Their seats are not just assigned by check-in staff when they arrive at the desk. The algorithm attempts to seat passengers in the same booking reference together.
I'm not sure how well it works if it finds a group of 27 pax under a single reference. Whatever logic they have worked into it, it seems to be fairly good at distributing the pax evenly between the three cabin bays onboard. Trying to seat 27 pax immediately together when seats have already been selected by other pax scattered throughout the cabin would be pretty difficult.