I wonder whether the total numbers are significantly lower on the weekends - i.e. if there's a selection bias going on for people who choose to use the internet.
It's probably how Net Apps manages to still get such high numbers of IE market share (they measure browser "usage"), when I think it's pretty obvious Chrome, and then Firefox are the most popular browser choices. But since a lot of people use IE continuously for work, that could raise "browser usage" a lot.