I saw a lot of this especially several months ago when schedules were more uncertain. I fly a lot of EWR-SFO and always look for the new polaris config flights. Sometimes, the PP seats were available as E+ and I could reserve it at time of booking. As other posters have mentioned, I think it's because they didn't want to commit to having a PP plane just yet, so left them as E+ seats. FWIW, it worked out well for me: 2/2 times that I did this, the flight remained a 777 and I got to fly in the PP seat :-) But of course that's not a guarantee that it will always work.