OK, my previous experience and reading the rules was always that as a Diamond Medallion I needed to use GUC's to upgrade to premium select at the time of booking or after. Then I would be on the waitlist for a 24 hour upgrade to Delta1.

Flying from ATL to FCO 2 weeks ago, the person sitting next to me got an upgrade when they were boarding to Premium Select, he was only a Platinum.... so I was disappointed I had used a GUC for the flight, not getting upgraded to Delta1. I was thinking I would have gotten the upgrade to Premium Select for free if he got it as a Plat.

Then on the return, I had basically the same thing happen, I finished number 1 on the list to Delta 1, took my premium select seat, and the guy next to me got an upgrade again. So then I am thinking coach must sold out and this was an Operational Upgrade. When I walked back to the use the restroom, I saw lots of empty seats in coach, including one person sleeping across the entire center row.

So why would a PM get an upgrade at the gate, if coach wasn't sold out? I feel like i wasted 2 GUC's on that trip. What am I missing, or not understanding?