Seattle's only 71? Geez -- people regularly move away from here because they can't face another dark, dank, dismal Seattle winter. We lost some great neighbors a couple years back for that reason. (And that's even before you get into corrupt leftist Seattle politics.)
Agreed...I would have expected Seattle to rate much lower than it did. My guess is that one reason for this is precisely what you mention, which is something I've noticed here too. People from all over the US read glowing magazine accounts of how Seattle is such a 'wonderful, livable city' and so people move here based upon that. After a few years of subjecting themselves to the 11 months of overcast and drizzle and one month of sun out of each year they get fed up with it and move back to Phoenix. Therefore, they don't stay long enough to become truly clinically depressed and get into the medical treatments for it....they simply leave and so they don't become part of a study like this.
Seattle is the worst place imaginable. I'm depressed here all the time, but was very happy living in Florida, middling happy living in mid-latitude states. Sunshine makes all the differenc in the world. I'm too depressed to move.