I'll double check the compression quality tonight. Maybe the compression artifacts are worse on specific images, or perhaps I made a mistake while spot-checking the compression quality.
Here's some docs on cwebp:
https://manpages.ubuntu.com/manpages/bionic/man1/cwebp.1.html
https://developers.google.com/speed/webp/download
On Ubuntu Linux it can be installed via "sudo apt install webp"
There are also multiple ways of setting the compression level of webp images during encoding. It's possible some quality targets are better suited to your images than others.