It wasn't. It seems that the image compression algorithm that GIMP uses for JPG just has a higher tendency to band at lower levels of quality. I just retried it

It wasn't. It seems that the image compression algorithm that GIMP uses for JPG just has a higher tendency to band at lower levels of quality. I just retried it to make sure that there weren't any mistakes, then attempted to make it look better using the advanced options to no avail. You're welcome to try it out though, the lossless images are here: http://jjcm.org:8081/webp http://jjcm.org:8081/webp I'm in the process of updating the images on the original blog post (I'm the author) using photoshop's jpg compression algorithm, and it should change the results a bit.