I've surprisingly seen a lot of upgrades on the PDX>SEA (and back) route. Of course, it's such a short flight there's no beverage service the only benefit is a bigger seat and quicker on/off from the front of the plane.

What anteater says is true... the processor does have a mind of its own. I've found as a single I often benefit and I know couples get the short end a lot.

Here's a recent DP: I was number 7 on the list with two sets of two in front of me. I believe they were at 2/3 and 5/6. Four seats open (all of row 1) and I was shocked that I got the upgrade. I looked back at the list and the two sets of two were still on it so they got passed over. Most logically, the set at 2/3 would have been put into a pair of seats and the two singles, 1 & 4 would get the remaining seats but they just left the pairs and took the first four singles.