Something that’s wrong about this blur is that it’s expanding everything that’s content on the page outwards, but not expanding things inwards from outside the fold as well.

This gives the illusion that there’s a fisheye or barrel distortion at the edge. You can see this most strongly when the images get to the top edge. The edges of the image curve outwards.

Apples blur doesn’t do that. It seems to expand the content outwards and then blur it uniformly from there. This means that the lines of the image do become fuzzier (as a blur would) but they effectively seem to be axis aligned, as you get the background blurring inwards as well reducing the effect of expansion.

There also seems to be some pre-emptive blurring that adds to that. If I bring an image near the top edge but not under the blur section, the top edge will be a blurred copy of the image, then fall into the background and then to the crisp image.