When we first implemented requiring players to demonstrate mastery by providing evidence of their learning an achievement reviewer could either approve the achievement or reject it. Rejecting an achievement reset the achievement and required the player to submit all of their evidence again and so we introduced suspending achievements, which allowed players to correct their work without having to do it all over again.
With suspensions, we were able to shorten the feedback loop and allow players and achievement reviewers to enter into a dialog. The need for rejection was basically eliminated but we kept it around until today.
Going forward, achievement reviewers will no longer be able to reject achievements.