I saw your power curve on Strava and it was like 313W for 45 min. But you climbed to almost 8500Ft. That elevation has to account for at least a few percentage loss of power. I know when I am fit I start to really noticing the power drop at about the 7000ft mark and even more so at the 8000ft mark.
Not only that it was your 6th straight ride in a row that week. Your FTP is probably closer to 340 than 320 you have it set at.