The populations are different, though, right? One could hypothesize that the set of all men in relationships past some age have median attractiveness higher than those who are single. The first set would contribute only to measuring total average. The second set would contribute to total average and also average on dating site.

It's also why the median job-seeker in a field is less skilled than the median person in the field of interest: the median person has been evaluated by at least one employer to be sufficiently skilled.