This is just free advertising for Comscore's real products. And the chart is about people surfing websites that use Comscore web tags. It doesn't give sales, or usage of all websites. Just usage of their covered sites.
Interesting numbers.
According to Apple, their total world iOS sales ratios to date are ~ 5:3:1 (iPhone, iPodt, iPad).
Yet Comscore sees a USA web usage ratio of ~ 3:1:3.
So, assuming that USA sales ratios are similar to world ones, it doesn't seem that web usage equates to sales very well.
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No, this just reflects the reality that it is far more enjoyable to browse the web on the iPad than the iPhone or iPod Touch so people do it more. Indeed I almost never use my iPhone for web browsing now that I have an iPad. And my Mac Mini's web browser is only used when I'm in the office doing work. Pretty much all casual browsing is now done on my iPad.
And the iPod Touch doesn't have 3G service so its obvious it would be underrepresented for web browsing as compared to say an iPhone. On the road, it's probably used more for music and games. Things that don't generally require internet service.