I, too, struggle a bit with these nuances. As an example:
1. I flew from HKG to SFO a few weeks ago, W fare. Applied GPU at booking.
2. Checked in online at T-24. I was fast, but not fast enough - appeared as #2 on the upgrade list in the app.
3. At T-24 and a minute, there was nobody on that list (accessed via "flight status").

Here is where I am getting lost:

1. The app stated "List only includes upgrade eligible customers that have checked in for this flight". At T-13, there were 6 people on this list (I was then #4 ), but at the top of the screen it stated "Booked 44, checked in 1". That does not make sense (to me).

2. At T-11, I was upgraded, together with the person in the #1 spot. The Wiki above seems to say that list IS prioritized. If so, why were people in the #2 and #3 spots on the list skipped (they were still on the list)?
At that point the screen showed 8 people total, 49 booked and 4 checked in.

Is it safe to assume that these upgrade lists is really meaningless? Perhaps its ONLY purpose is to show you that you ARE indeed on the list?