For me, the difference is chamois and cut. My midrange shorts have a generic blob of padding on them (out of a lack of a better term) where the higher range ones are very contoured. The higher range ones also fit better. I actually have some older Performance shorts (lower than midrange) that have a seam in the chamois

Check out the TE shorts selection because you can click on the chamois and it'll pop up with a picture. It's pretty darned cool, actually.

The best thing is just to get advice then see what works for you. I have some not-quite-high-range shorts that are really surprisingly comfy though I haven't gone over 30 miles in them.