^Not sure but it's probably because there's only 7 slots & you need more flexibility in the top end. I don't use them though; I thought you asked for a link. Just try some slower kernels & see if that helps.
Below is another but different link for the sapphire kernels that has more of a selection & yes sapphire kernels do go from 125 or 250 to 400 so do p3 kernels.