The upturn makes no difference at all. It's fixed to the rubber mat, so it's only as good as the grip between the mat and the ground - if the mat slips, the ladder slides. The only thing the upturn does is stops the ladder sliding on the upper surface of the mat, and if it were possible for the ladder to do that, the safety device isn't much good!!
The only thing any of these "safety aids" does, is increase the surface area of rubber in contact with the ground, and if the ground is slippery, it makes no difference how big an area you've got, the mat will still slip.
The danger with "safety aids" is that they can give a false sense of security, and lead to taking risks you wouldn't otherwise.
Rubber mats, ladder feet etc work because there's friction between the rubber and the ground, increasing the area doesn't increase the "co-efficient of friction" - i.e. when the point is reached where the grip between rubber and ground gives way, it WILL give way, regardless of how much area of rubber there is.