Interesting tool. I am not sure if I like it or not. It doesn't look too good for most sites though.
I was aware that some people still uses a low resolution but I didn't think it was this bad.
However I interpret it the way that they have taken into account also parameters like when visitors don't have their browser window expanded to the maximum or when visiting with a mobile device (and the site is not specifically enabled fior that).

Something to consider anyway. for your own site!