@ElizaWy, I'm not the author of those tilesets, only the copyleft holder. I've asked my friend who created those and he said a couple things:

• He was trying to replicate the details he saw on a painted background.
• It's based on a shading technique for painting regular walls (not pixel art).
• He used more colors than usual because that was one of the first tiles he created and so he wasn't sure how many were necessary.

I asked him for the link of the painted background he used for reference, but looks like he can't find it anymore. We appreciate your curiosity <3