...I think in a leadership contest 50%+1 is considered legitimate while in a leadership review approximately 70%+ is generally considered to be adequate. Labour seems to be stuck in a stalemate between these two requirements. I guess something will eventually have to give.
The reason for the difference is that a leadership review does not lead to the leader resigning necessarily. It does lead to a leadership election, wherein the winner needs only a simple majority of votes to win. So the two situtations are not really comparable.