Not really joking. I think what they look for, and how strictly, varies by school. Nationally their rule is only 10% of a class nominated, but they're all about showing some love for humanities/liberal arts. For example my school states:
"In making its nominations, the Selection Committee reviews each record looking for evidence of both breadth and depth of interest in the liberal arts, as demonstrated in a student's work at Washington University. One record, for example, might be for a 4.00 science major who shows few interests outside the sciences--just the minimal set of distribution requirements. Such a student is clearly outstanding and deserves special recognition of some kind, but he or she probably would not be nominated"
Well, I guarantee you there are some seriously brilliant people here who knew they wanted to study Chem/Neuro/Physics/Etc and only took the required non-science classes. So while PBK elects only ~7% of senior students each year here, it's hard to say it's the top 7%.
Latins is a similar deal, in order to be eligible for it in some departments (including Bio where most premeds are) you have to do a ~40-60 page research thesis which not a lot of people do. So Summa Cum Laude puts you in the top 15%...of heavily research-oriented students.
In the end honors are cool and all, but if someone can come in loving science, make a 4.00 studying science material, and get no PBK or Latins, I become skeptical of just how impressive the honors are. Schools that base it just off GPA are much more legit, imo.
All that said, I'm still forking over the $75 for my invite, because some people like to see it on your resume/assume it just means you were near the top of your class.