It appears that only BlurHash is using base83. I imagine the base83 encoding is being used in the table because that is what the library returns by default.
As to why everyone else uses base64, I figure it's because base64 is what you'd have to inline in the URL since it's the only natively supported data URL encoding.
In other words, in order to take advantage of the size savings of base83, you would have to send it in a data structure that was then decoded into base64 on the page before it could be placed into an image (or perhaps the binary itself). Whereas the size savings of the base64 can be had "with no extra work" since you can inline them directly into the src of the image (with the surrounding data:base64 boilerplate, etc.) Of course, there are other contexts where the base83 gives you size savings, such as how much space it takes up in your database, etc.
As to why everyone else uses base64, I figure it's because base64 is what you'd have to inline in the URL since it's the only natively supported data URL encoding.
In other words, in order to take advantage of the size savings of base83, you would have to send it in a data structure that was then decoded into base64 on the page before it could be placed into an image (or perhaps the binary itself). Whereas the size savings of the base64 can be had "with no extra work" since you can inline them directly into the src of the image (with the surrounding data:base64 boilerplate, etc.) Of course, there are other contexts where the base83 gives you size savings, such as how much space it takes up in your database, etc.