Finals tend to be low scoring because they are high pressure and intensity games. Everyone goes harder at the ball and player, working harder to shut the opposition down and finding it harder to dispose of the ball well. As a result, there's very little clean play where there are easy scoring shots and so there's fewer goals.
More defensive sides like Sydney (and us) score even less and struggle even more to find scoring avenues. Teams that have a good attack will still be able to find goals but at a lesser rate. If you're lucky to kick 10 goals per game in the first place, it's going to get even tougher for you come finals.
That's one reason good tall forwards become very important in finals. If a tall is good in contested situations and little or poor supply, they can really turn a game. That's where having a bloke like Jack Reiwoldt (or I shudder to say Josh Kennedy) makes a huge difference to a side. You pick up a gun KPF early in the draft or not at all.
