Easiest way to think of it: consider the position of the bottom right corner. It can be put in any position with a resize, so the line doesn't have to be straight.
The description didn't mention taking any screen obstructions into account, so I don't think you're correct; more likely it's the effect of overlapping screen-widths and heights. I think the answer is mobile browsers and netbooks.
The bumps disappear after the 350 vertical and the 600 horizontal line, so the screens making the difference are around that size.
Some small percentage of people have narrow but tall browsers (non-iPhone mobile browsers), so that's the little bump at the bottom-left of the 99% band. And other people have very wide but short screens (netbooks), so that's the bump at the top-right of the 99% band.
