at least when the big story included him.
I think they go like this
Who has the biggest story this month. If they can't decide then step 2.
who is moving the most merchandise. This is either Cena or Punk, depending on how close to a great "Pipe bomb" we are at the present time, for example, in the month after Punk's infamous worked shoot, his merch sales beat Cena, the first time anyone had sold more product than Cena in years. he was also (if memory serves) on the cover either that month or the one right after.