You are not selectively remembering stuff. You are just perceiving it incorrectly.
It’s true that Souths were ending their sets farther down field that the Sharks. The key difference here was that the Sharks were giving up piggy back penalties, 7 tackle sets, and not ending their sets well.
Souths weren’t making 70m per 6-tackle set. They were making 50-55m. They were just starting their sets farther from their own try line, or getting an extra play to make another 10m.
Last bit from
@bort is correct. If you keep giving the opposition an invitation to your 20, at some point they will force you to work it out of your end in to the teeth of swarming defence.