I don't get the whole door slamming thing either. The last car I had to slam the doors on to get them to close with the windows shut was my first car, a '68 Beetle. Every car since has had a pressure vent installed, typically behind the rear bumper.

 

lmao, I think we have the same father in law.

 

The same thing happens to me too, I just attributed that due to the fact that I have larger and heavier spare tire on the back. I noticed it makes it hard to move the tailgate hence more force is needed to slam it shut.