I don't know if it is almost intentional on UA's part, but I have noticed that right before the window they do adjust R space often, most of the time reducing the # of available upgrade seats despite F inventory remaining the same.

I am on a rock bottom fare for this Saturday... ORD-SLC, the first flight, it was F5 R4 when I went to bed around 10pm last night, this morning it is F5 R3. The kicker is, I have a companion with me, so if there is only R1 [assuming I am not the only 1K on a rock bottom fare] you know what will happen.