I think showing the pax uplift with the lowest possible configuration is lesser risk scenario than the other way around.
If the system would show, for example 150 pax out of 210 for the best configuration, someone who has bought a 767 with the lowest configuration from UM would have no clue what to expect from his bird. Will it carry 120? or a 100? or 70? The economics of the route will vary wildly when we get to those extremes.
The current system gives the least amount of pax that any 767 can possibly carry, which is fair in my personal opinion.