I believe that clangers don't have to be disposals. Getting caught HTB, deliberate out of bounds, dropping an uncontested mark, things like that. Disposal efficiency is not the same as "was that a good kick". I can't remember the exact definitions but things like a kick over a certain distance (40m?) that ends in a stoppage is recorded as efficient. So is kicking a point from a set shot 10m out directly in front, because it's a scoring shot.
Shanking a kick, chasing it down yourself, then getting caught holding the ball could well be recorded as an effective kick (because the team kept possession) and a clanger.
Someone will probably know for sure what the current definitions are and correct me.