In very hot weather the same flap may grow so large that it can't swing freely closed with the same result. (Any claim that a flexible flap material won't shrink or expand with temperature change is bogus).

Some manufacturers use a brush type weather strip against the edge of the flap to "maintain the seal" in the face of shrinking or expanding flaps. While these weather strips do eliminate the "line of sight" problem (you don't see the gap), they still pass air. You can confirm this by blowing against the joint and feeling the air against you hand behind the joint.

The Endura Flap incorporates a patented 'bellows' design on the flap edges. These bellows move freely in and out between the flap and the frame and permit a magnet running the full length of the flap on both sides to maintain tight contact with a corresponding magnet embedded in the frame. A floating magnet bar across the bottom ensures a tight seal on the fourth side.

Here's the grim reality of pet doors without this design...