If you look at the flight status seat map (not the booking map), UA928 has been Polaris pretty consistently for the past month or so. Today was the first time in a long time it wasn't Polaris. Noticed it was coming from AMS inbound at least once, not sure where else they use it. I'm on UA 958 next month. Depressingly, that hasn't been Polaris at all. Not sure what the route assignment criteria is - certainly not random. I think two more Polaris 767s are suppose to arrive shortly. That should improve the coverage odds.