I think I'm pretty good with them. But I don't understand why you're so upset about them.
The incoming solar radiation ranges from roughly 1370W/m2 at the tropics to virtually zero at the poles (depending on the time of year and the height of the sun in the local sky).
Looks like the authors of that graph have used some mathematics to figure out an "average" figure for the whole earth and used that in their modelling. I really cannot see the problem.
