You adjust your definition of "win" to the combo you're playing.

A healer with 75 solo PKs won big.

An assassin with 75 solo PKs...not so big a winner.

That the assassin class is likely to get more kills than the healer class doesn't mean you're not "playing to win" if you choose healer. It just means you're playing a different game, so to speak. You're playing the "see how many kills I can get as a healer" game as opposed to the "see how many kills I can get as an assassin".