All default dimensions in Minecraft use the seed "0" in their dimension file, which is why their generation matches the world seed. However, if you try to add or modify a dimension and use the seed field, seed "0" is treated as an actual seed for the dimension and not a seed replaced by the world seed. This issue becomes an even bigger problem because excluding the seed field from a dimension file prevents the dimension from generating, causing all modified/added dimensions that use the same datapack to generate with the exact same terrain.
To reproduce, create any dimension file that uses the seed "0", and make two separate worlds that use the same dimension file. Even though seed 0 is supposed to randomize the seed, both worlds will generate with the exact same terrain.
Note that the world spawn and configured features are randomized, but teleporting to the same coordinates will give you the same terrain and biomes.