The damage difference between demolish and snap punch should not be that large, it's likely that a few of your demolish ticks were crits.
Considering that Demolish doesn't gain from Dragon Kick's debuff(Correct me if i'm wrong) then the potency difference between snap punch and a full demolish should be a whopping "2".
Edit: "42" - I always think 18/3 is 5 for some reason. Bad brain is bad.