I recently had to change an upcoming trip to originate from a different airport. I called Delta's Medallion Reservations line to do it, they waived the change fee and quoted me the fare difference, and confirmed what credit card I wanted to pay the fare difference with. The first leg of the new itinerary is on Delta metal, and I've already been upgraded to Comfort Plus. It was amazingly smooth. Until....

The second newly-added leg is a Korean Air codeshare. The Delta website said I needed to call KE for a seat assignment, so I called KE's customer service line. The agent there said that a) I needed to call Delta's Codeshare department to get a seat assignment, because it was a Delta ticket; and b) my ticket and my reservation didn't match, so I needed to get my ticket reissued.

I called Delta Codeshare. (Turns out that's supposed to be partner-airline-facing, not customer-facing, but the agent said that Korean in particular gives their number to customers instead of calling directly quite frequently.) The agent there said that she couldn't do a seat assignment either, because my ticket didn't include that flight yet, so she needed to fix (b) first. She said the person who changed my reservation should have reissued my ticket as well, but clearly hadn't. I checked, and found that there was no pending charge from Delta for the fare difference. This agent was able to waive the minimal fare difference as well and reissue the ticket directly to the already-reserved itinerary. All is well, and I'm glad we found this a few days in advance rather than when I arrived at the airport for my flight.

This leaves me a bit mystified on the gap between reservations and tickets, and I'm hoping the experts here can enlighten me. I'm familiar with the reservation existing before the ticket does when I book through my company's travel agent -- they create a temporary reservation pending approval from my manager, then issue the ticket after I have sign-off to take that flight. But this seems to be something different. I haven't encountered the situation of a ticket and a reservation containing different schedules before -- once it's booked, I've seen them as a single thing (though still changeable). Can someone explain how this process actually works, since clearly my mental model is incomplete?