While I cannot explain the full details and will leave it up to an actual airline emp (Zsmith2 or DLres), it is possible to have a reservation without a ticket.

A reservation can be made and changed based on fare rules at anytime. Once a ticket is issued, then there is an e-ticket (006#) that matches the reservation and you can fly. If you make a change to that res without a reissue or revalidation, the e-ticket will not match and you get the red light when trying to board a plane. I have had it happen most often during IROPS even on DL. For example, they reroute me through a co-terminal due to IROPS and didnt revalidate my ticket. Award tickets also come to mind but it happens much less often due to the immediate reissue requirement by DL now.