The Celts have the 8th best offense, the 4th best defense, a +5.6 point differential, and are tied for the 8th best record in the league.
They're top third of the league but a step below the elite.
They kill bad teams regularly but drop games against good opponents. That shouldn't be too surprising. Most good teams do that.  
But they've also had some bad injury luck and their supporting cast is uncommonly shaky for a team expected to win 55+ games.  Not due to lack of talent, but due to a lack of players with well defined roles and stand out skills