That's because in the last few years we've been trying to expand our game. We've scored those tries because we found a way to create space and had the speed to do that.
In the NH we revert back to the traditional style of play, due to the conditions. And that is the way we know how to win. It has absolutely nothing to do with altitude, because unlike KeybaordWorrier's petty attempts, we actually have stadiums in South Africa at Sea Level!! (Durban, Cape Town, Port Elizabeth), where we have beaten plenty of teams...
In this world Cup we used a more conservative approach because we found that there were too many gaps being created in our defensive lines during the Rugby Championship, and because of it now being knockout rugby, we rather focus on winning the game than getting bonus points...