Thank you for the pictures and information. Very helpful.
The mats you have don't look like mine. My liners are continuous over the location of the retaining posts on the driver's side, and the same for the passenger (even though there is no factory post on that side). They both have velcro underneath to accept some sort of Weathertech-supplied retainer.
It's hard to describe without pictures, so I will try to provide some when it's light outside. I'm curious to see what's wrong with my setup. This doesn't make any sense!