There was originally something wrong with the Leaderboards when they didnt upgrade at all after the month changed. That was corrected but after that I was still seeing alot of people above 5.0 average.
Though this can naturally be possible because of the other addition to the forums
For example if someone posts a dicussions and another person votes using the "I have this problem too" button and someone answers the discussions correctly and that post is acknowledged by the discussion starter as the correct answer (or I guess endorsement for Cisco or Cisco Designated VIP will also do) then the person that provided the answer will get a single 10 point rating (or even more with multiple votes, which I havent seen happen on the Security section to this day) So that might bring the average of a user up beyond 5.0.
Though I have to say that there is still a lot of people with that past 5.0 average which makes me wonder that something might be wrong as Paolo has said. I cant remember if I have answered a "voted" discussion this month but dont think I should still have 5.1.
With regards to the Correct Answer section there was another discussion where I commented about that too and Dan gave an answer
Heres a link