The deltas are just differences in quality.
I guess what you really want is conditions to be not based on the supply, but rather based on co-occurrence inside a single deck?
I'd really like this. Given two cards, A and B available in a kingdom, how have players historically fared buying both, just one, or neither? If there's enough data, it'd be nice to be able to see it broken down by opponent purchases, too. So, buying A against an opponent who buys B, or buying A + B against an opponent who buys only A.
It seems like that alleviates the confusing results theory mentioned before. Just now I was looking for interactions between Philosopher's Stone and Ill-Gotten Gains. The delta is near-0, but I realized that doesn't mean much because people rarely gain P. Stone when it's available.