They are basing those numbers on their own personal websites logs. I know 10s of 1000s of users that have never visted the w3schools.com website. A better stat would be published by Facebook or Google.
It is more or less useless, cause it only lists the most popular resolution, but could give you an indication of what your target audience is "most likely" using (unless you are like me and are color blind, makes it a bit harder ). Funny to see that 1366x768 is the most popular resolution in large parts of Africa, did not expect that.
Not many people now design with 800x600 screen res in mind these days, instead shifting over to 1024x768. With 960px being the usual most go for on fixed width. 760px is too narrow when coming to forums and can cause cramping with the layout e.t.c
I guess if you run a business website selling something, it can still pay off to use 760px fixed width to cover all, still makes sense in that respect. But does suit website design mostly and not forum boards.