It does make sense when looking at system percentage install base.
The PS2 currently has about 120 million consoles in circulation. The Wii has about 1/3 of that (which is remarkable) with about 40 million consoles in the public's hands. The usage percentage bears that statistic out, rounding to 32% and 13% for the PS2 and Wii respectively, its very nearly the same 1/3 ratio. Especially because the PS2 and Wii are the two most "casual" oriented systems, one would expect this similarity in sales to usage ratios.
The Xbox 360, for example, while selling about half of what the Wii has, has a higher usage percentage most likely because of the type of gamers who own 360's.
I think its important to recognize that the total percentage of usage minutes for last gen consoles is about 45%, which tells me that current gen systems ought to be around longer than the standard 5 year period.