There's a few reasons for wheel arches like that. Closing angles behind objects of more than.. I think 6 degrees cause flow separation, and are just not worth the material you make them of. What you're seeing there is the wheel flare being carried back to a convienant panel line, then instead of trying to re-attach flow, they're going with a sharp cut. This causes what flow there is to separate cleanly. The area behind the flare is also open to the wheel volume, providing an air supply to fill in under the flow separation. This acts to suck air out of the wheel well.
Some cars go so far as to completely eliminate the wheel arch behind the wheel. It's a very nasty place to try to put any ~useful~ bodywork. So if there's the option, they often just eliminate it. That sharp edge along the top of the trunk, serves the same purpose, flow wise. Having a sharp edge to release the air attached to the body helps reduce drag significantly.
I need to buy some sheets of lexan. I want to see what putting an underbody on my car will do... Sadly most of the classes require "complete" rear bodies on the cars, making under body air evacuation tricky.