That's an interesting question. I looked into what Acrobat does when you use the highlight tool on an area of text - you can do the same but you have to zoom in after highlighting to see what happens.
Each highlight has these curved ends. But each lineof text is separately highlighted. Suddenly, the meaning of QuadPoints becomes clear. Rect is the overall rectangle, but QuadPoints is a long list of quadrilaterals for each separate piece of text. This is how Acrobat's highlights, have a ragged right edge on unjustified text, and allows the highlighting to start or end in the middle of a line.