Though many have low post counts, most of 'em have been members for around a year. A couple of 'em predate most of your join dates. The one full name was likely a mail-in entry. Congrats to the winners!
Notable how many of them joined in 2011.
Either membership increased significantly in that year, or the half-life of a member is less than 2 years. Bit of both, I'd guess.