Datapoints:

UA1620 on 3 May - DEN-PHX.
R/RN = 9.
Seat map shows 16 of 20 seats unassigned.
At 96 hour window, 1K does not clear on a K fare.
Reservation is updated to Complimentary Premier Upgrade Requested.
"TOD" - offered on reservation page $339. (as an aside, while I keep seeing TOD, my personal experience is that it's never less than $200 on any of the tickets I manage).

Presumably, CPU processing did occur, but this reservation wasn't upgraded. Now, due to lack of transparency, I don't know if the fare had been say, an H or a Q, would it have more likely cleared? Has CPU processing improved to the point where a higher fare clears sooner than a lower fare? If CPU doesn't pull from R/RN, what is it pulling from? Is UA Insider able to provide any light on this subject - since the stated manner in which CPUs are to process is not how they are happening and it has clearly gotten worse since 3/3?