Reading the descriptions carefully, here is how I understand the 5/6 rankings:
5A: Simple two-hit combos, basic stuff, some AT's used.
5B: Combos that deal lots of damage (40-50% or more). All generic AT's used and few advanced AT's
5C: Spacing has improved
6: Amazing people.
So from this, 5A is attainable by most people, 5B are for the mid-level smashers (or most of the lower level Kaillera community), 5C is for the higher level smashers (mid-high level Kaillera community). Using this logic, most of Kaillera is 5B, and the better people are 5C. So people overestimating their placement may not be true... it may just be the way the rankings were described leading to a vast majority of the community being in 5.
If you think about it, there is no real way to fit people in their own ranks. People go through different paths when playing this game. People coming from melee will be low on the rankings (maybe around a 3) in terms of their playstyle, but a 5A in terms of AT's (since L-cancelling, dash dancing etc are in both games). There are way too many variables to consider to make a completely accurate list.
I personally think that a person's rank is determined not by how he plays, but by the way he sees the game. Beginners see it as 'try to hit the opponent', and the higher you go, the better the way you see the game. I notice this in my local smash club, as I see the game differently from other people, which gives me an edge (and differently skilled people see the game differently).
Making an actual list in this sense would be impossible, since there are literally infinite ways to see the game, but different ways have different advantages.