I can only speculate that the 3:1 ratio is put in place so that dev can keep the price of omniseeds the same. As granite is a more valuable resource, the thought was probably that refills need to be more expensive. So only getting about 80granite for omniseeds unbuffed. Some input from devs on the reasoning behind that 3:1 ratio would be nice. I would prefer to see a granite specific refill only because people will be confused about how much they get for their money.
Thanks