This height above the ground dimension is called "rise" and lower is better other things being equal.

The point is illustrated above. The first flap is wide enough for our dog. But when you install it at the proper height, the 'rise' that the dog must step over is quite high. Older dogs, infirm dogs, a smaller second dog all will find this to be a problem.

To get a lower rise in the same brand, you get the next size up which is taller.  We're showing this flap in the middle above.  Now the rise is fine but you don't need all that width!  We're back to the security problem on the preceding page.

Finally, in the third example, the Endura Flap gives you a lower rise while maintaining the top of the flap at the withers and without extra width that you don't need.

Of course, if you do need more width, you get it by getting a larger size. Chances are, though, if you need a wider door, you'll want an even taller one as well and that's what you get with the Endura Flap.