They "mine" the product from 3 different locations around the US.
They amend and the soil mined can shift. They don't catch that and correct for it enough.
Not to mention the differing needs of that soil from the 3 different places. Requires 3 different amending formula's to get the product as "uniform" as they can.
They do do a damn good job for what their doing and the size of the operations doing it.....
A good "gardener" reads his plants needs as they grow and adjust at his end to cover their needs.