I've been on this flight a few times and in each case it is a very "active" flight. What I mean by that is you have people connecting in FRA to places all over the world. India, Russia, Africa, etc. So it is has quite a large number of connectors.. much more than your average UA trans-Atlantic flight.

I think this is why it makes upgrading particularly hard. Tons of *G the last time I did this flight, last week.