There seems to be no rhyme or reason to how this is done ,or when they choose to run EUA or not. It seems a safe bet to assume that this is how they wish for that to be and in its current format will not be transparent by design.I can really only make educated guesses at best from data I gleam from looking at the available buckets,seat charts,and rules they have given us for each elite tier.When all is said and done their lack of tranparency keeps their revenue management in the driver seat and me guessing about how many seats are available,when ,why,record problems,and all these bucket questions.
I do sometimes get an upgrade at the 5 day window and really have no clue why. I also get a fair share at the gate and some battlefield ups on the plane. I like these the least as I have to find my carry on, move it and don't care for everyone looking at me.
The most important things I look for when making a reservation are the following;
1)Aircraft (no jungle jets and always pick a flt with the biggest FC cabin)
2)Some routes will always be a crapshoot (transcons ,Iah -EWR/SFO/LAX/Calgary so I follow step one above and hope for the best
or use miles from the get go to upgrade hard routes
3)When booking I try to book a higher fare class than X or U so my upgrade chance is better
4) when relying on a EUA that has not come through I make sure to check in on line at the minute the 24 Hour before flight check-in becomes available so I am on the top of the waiting list ( I hope ,but this too is not transparent)
5) I try to get to the gate early if not EUA'D and chat up the Gate Agent