I don't think you're quite getting how reforging works.
Basically you bring a piece of loot to the reforger. You take a stat that you don't want and convert it into something you do.So for rogues that tends to be Crit or Expertise into Hit or Mastery, but it doesn't have to be. You can turn Crit into Haste if you want or Expertise into Parry, but we reforge into hit because it's the best stat for us to reforge into.
It stops becoming the best stat to reforge into after we hit 17% hit chance. Once that happens Mastery becomes the better stat and so we start reforging things into mastery.