Overworld like dimension doesn't spawn custom grass right.

I am working on a mod that uses a custom type of grass and dirt for it's over world. I have it set so that the biome has a layer of the grass at the top followed by dirt, then the stone. The thing is, it just skips the dirt, and fills the whole thing with the custom grass. Also i have it set so the secondary filler block of the dimension is just the dirt.