Taken right from the GenRight product page:
"This simple design is also adjustable forward or backward 1" once welded in place."
https://genright.com/products/rear-coil-spring-correction-kit.html
So they are just making that up? You can adjust it, clearly. What for, I'm not sure, but it's obvious they're adjustable.
They are adjustable not for lift height, but for half way attempts to lengthen the wheelbase without a frame stretch.