I think you're mixing the width's and height's of the pieces. If you notice, all of the pieces (including the angled section and monitor shelf) have 26'' long sections, which will be the width of the cabinet. The 24'' and 22'' measurements are used for the height of the pieces and are all accurate in the layout drawings. When I built my cabinet, I followed the instructions to the letter and it all came together perfectly.
edit* just re-read what you said and I may have been mistaken about where you were confused although I'm still not entirely clear if I understand you. Anyways, If you look at the layout on page 23, you can see that in the sections where the speaker panel and angled section are placed, there are 2 extra inches that need to be cut off.