I think that it is broken into 3 sections. UserID/Session|Stats|Login Time.
My Guess is that the first one is the Session Key that pertains to the user, and the second one might be to distinguish sessions from different computers or a tracking key.
I could be wrong on my first guess, and it might be that it is UserID|Preferences|LoginTime if StackOverflow they have chosen to keep some preferences stored in the cookie, so it doesn't have to store any data in a PHP Session-Like (I know it runs on ASP) system, and then just goes and grabs the rep everytime.
Then again, I could be wrong on my second guess too.