Does not really surprise me. Reflects a couple of things:
1\. Speed of the game was quicker &;
2\. 2005 was the year of the fairytale for both Wests Tigers and North Queensland.
You will find that both teams are generally viewed by many fans as their "second" team. In 2005 Sydney and Brisbane would have rated through the heavens for this reason.
Probably should also consider what had gone before them:
2002 - 2004: Roosters in every GF
and then what followed:
2006: Melbourne would have dragged ratings
2007 & 2008: Melbourne & Manly…..a ratings nightmare.
2009: Again Melbourne
2010: This is the one GF that could have rivaled 2005 with two Sydney teams. However, they lose the Brisbane market
2011: Manly and NZ....almost as bad as Manly and Melbourne