Im not as well versed in japanese stuff but ive seen it alot in the american cards. Usually varies between printers depending on plates, ink, card stock, print runs etc.
Could it also be heat and humidity? I come from a very hot and humid country and the jp set had to be imported. Maybe one vendor stored it wrong or is that less likely to cause this?
Nice! It makes me realize I prefer the darker variants. They look deeper to me, while the lighter prints seem overexposed But I guess it’s all about personal preferences (I love that blue and pink Pikachu !)