Thanks for the reply Ethan!
I should not have said the majority of the flock is Lessers - clearly there are a good number of Greater Scaup around. But there is also a fair number of Lesser Scaup around. I think you are probably right that I saw several obvious Lessers in a group and made assumptions I should not have about the rest of the group.
Before this discussion, I genuinely didn't realize that Lessers were unusual in the area in summer; I should've been more careful and looked up the historical status. However now that we know there are several Lessers summering in the area, I think a reexamination of summer scaup status is in order.
I also went back and changed most of my sightings of this flock to Greater/Lesser. I think I will probably be using this option more often in the future, especially with ratty summer birds, or otherwise will pay closer attention to each individual in the flock.
Also, its possible that, among the larger flock of scaups moving around in the general area, there is at least some degree of associative flocking or subflocking. Usually there is at least some of this in winter, with the Lessers preferring to hang out with other Lessers, and the Greaters preferring the company of other Greaters. (Of course they often mix together as well though).