This weekend I noticed a couple oddities with the standby and upgrade lists and I'd like opinions regarding what happened. None of this affected me, so these are just observations.

1. A flight had lots of empty seats (according to EF) and always more available seats than people on the standby list according to the DL app, but we then apparently left with a couple people still on the standby list and not cleared for the flight, despite the app still showing something like four seats available. At first I was envisioning nonrevs being left at the gate because they had annoyed the GA or weren't dressed properly (which would be hard to do according to current rules for a seat in coach), but then I started thinking that it could have been someone with an E class ticked who no showed or had a late connection. However, if so, shouldn't the person have been confirmed onto the flight by the GA (and thus show a seat assignment on the app) and then simply fail to board?

2. An international flight with again lots of empty seats, where D1 availability went from something like 7 to 3 within about six hours of departure and PS availability went from 19 to having the entire PS seat map filled. No one was on the upgrade list on the app for D1 or PS, but there was a long list of folks on the standby list, consistently slightly under the number of available seats according to the app. As departure time approached, people were cleared into coach seats and the D1 cabin was filled. However, with no D1 seats showing as bing assigned to anyone on the standby list and never anyone on the upgrade list, so could that mean that these seats were filled as cheap paid upgrades at the airport? I guess there could have been a VDB on another routing being rebooked onto my flight or something similar to account for only a small number of seats. The PS cabin is harder to understand. According to the app, no one from the standby list was given a PS seat and there was no one on the upgrade list and no upgrades listed. Yet those 19 available PS seats were filled--in fact, when I peaked into the PS cabin to see for myself what how the new cabin looked, it was full--and there was never an indication that the seats were assigned to anyone on the standby list (and no one was on the upgrade list). Yet according to EF, the PS cabin went from being less than half full to being totally full a couple hours before departure, which would have been before the GA arrived at the gate to do any upgrades or seat assignment. These shouldn't have been OPUPs either as coach didn't seem to be overbooked. The standby list grew as departure approached, but I don't think there ever were more standbys listed than the number of available seats. [I suspect some nonrevs flocked to the flight at the last minute for a fun trip probably in a premium cabin, with the odds looking great for at least getting PS.] I was asked to bid for a VDB at check in but when I asked at the airport, I was told that the flight wasn't overbooked or oversold. AFAIK in order for DL to need to do OPUPs, there first would have been more people on the standby list than available seats (which I assume would show on the app as the total number of open seats, D1 + PS + coach, right?), where only those who are confirmed on the flight count for this, not nonrevs and normally not SDS (although some mercy might be shown if weather IROPs were expected, etc., but this was an international flight where no one should have been doing SDS, so that the standby list would consist only of confirmed passengers without seat assignments (including E fares) and nonrevs.

Both cases just looked very strange to me. Any insights? I'm curious about what I observed. I know there can be last minute ticket sales, but 19 PS seats?

If the flight was payload optimized, then some seats would need to stay empty and lowest priority nonrevs would be left behind.