Hi flutefish -
Ah, thanks for catching that. This is indeed a bug but its just a display bug, not a data-related one. The problem is that I've connected the "Last active" field to the wrong database field. Instead of refreshing the time/date for any page load, I had it set so that it was only refreshing when a user either logged in or was active on the forums.
Everyone listed in the September competition has indeed been playing this month - the handful of users who show last game dates in August simply haven't had to re-login and haven't browsed the forums since that date.
I've fixed this bug now so that the last active date will properly update whenever any page is loaded on the site while you're logged in. So most if not all of those old dates should disappear once these players come back to the site and their timestamp is updated. Until then the page data will be a little screwy.
Sorry about that, but thanks as always for the catch! :-)