I can't for the life of me figure out why either, but I had come up a bit short on certified seed potatos a few years ago, so just finished the row with about 8 hills of the 2rd year potatos. They all got treated the same, watered the same, and when it came time to dig them, the certified were all nice big, and lots of them. The 2nd year potatoes were tiny little things. It doesn't make sence to me, but I never did that again. I thought it was the same genetic material, but maybe not.
