I don't think that it's just the physicality at the breakdown, it's the decision making.
You watch from now and see if you can see a pattern, I've noticed that the position of the tackler is the key to whether they commit. As soon as the NZ sides have a tackler on the wrong side they pile in, which does two things, disguises the tackler not releasing/rolling away (there are usually bodies everywhere), and gives them a much improved chances at turn over ball, as the ball can't be placed at the back of the ruck and is now a 50/50.
It's smart play, not necessarily within the laws, as in most cases (the majority at least) you could penalise the tackler but not the players coming into the ruck. And the reading of how the ref is playing the ruck and the decision making of the forwards is the key. If the ref hesitates with the tackler not rolling away it's a tough decision to then come back after the ball is turned over and penalise the side that has clearly won the breakdown.