The cowboys centre ran into the back of Matai in the first one. I can see how this one may have been ruled
The second one was the perfect use of a decoy runner. The other player had commited to tackling Watmough, and if he commits to tackling the wrong the player, it isn't an obstruction, full stop. It has nothing to do with the offensive player making contact with a defensive player, the rule is about a defensive player commiting to tackle someone and being obstructed to do that. He wasn't, Smith committed to watmough, stewert scored.
Moot point though.
It's not like Manly needed to put more points on the cowboys. How a team with a negative for and against makes the preliminary final is beyond me.