IndustryGamers, EEDAR analyst Jesse Divnich suggets that the average score for Wii games may be improving because a significant portion of titles aren't being reviewed. Strictly comparing January to June, in both 2008 and 2009, the analyst group graphed the total number of titles released for the three major platforms and the percentages of those titles that were not reviewed (see above). The number at the bottom of each bar represents the number of titles released for the platform in the stated time period; while the percentage at the top corresponds to the quantity of those titles that was not reviewed.
Divnich recognizes that "while the Wii has experienced a rise in average review scores (from 60 to 66 in one year), this has come at the benefit of critics ignoring some titles that could have possibly dragged down the Wii's average." However, it's possible that such reviews could have also raised the average score -- we know, that's unlikey, but still. Additionally, the report notes that the unreviewed Wii games weren't "core-targeted titles," which also holds true for unreviewed games on Xbox 360 and PS3.
Divnich maintains that Wii games are getting better, but he concludes, "It is just coming at the cost of not reviewing the increase in casual and low-budget titles." He believes that a similar issue could appear occur with respect to PS3 and Xbox 360 as the consoles' casual demographics increase in conjunction with the continued growth of their install bases.