Thats because
1. The Roosters played in the final and therefore had more games
2. When you've got poor defence the opposition have more opportunities to score, better chance of putting on pressure/momentum, and therefore, gaining more possession of the ball, thus limiting attacking opportunities in the first place.
3. Poor stamina also leads to 2.
4. I'd say( correct me if im wrong) that the majority of the Rooster's tries last year were against teams that they had already hammered defensively early in the game, thus tiring them out and creating weaker/easier lines to attack. I will reiterate that the reason for the Rooster's success in the last couple of years has been their ability to run down teams that they have tired through the employment of the on-rushing defence, a format of play unseen and one that would probably have been disallowed in previous years.
The Knights, on the other hand, scored the majority of their tries early on when the other team was still fresh, primarily through their attacking prowess, and unlike the Roosters, tired quite quickly late in the game. As I've said before, the first round match showed that this years Knights probably has better stamina and defence, and I will say again that this year we've probably got a better chance of winning the premiership than the last time we won it-2001, as long as we dont lose the attacking prowess of recent years.