He's saying it is relative to pop. So for example if there are 10,000 Flick T206s and 1,000 T207 Chances then the % of cards with that respective back is roughly the same.
I see the other side as well, which is why I said the rarity is equivalent to Uzits. If there is only one known example of a card and it therefore only has one back then the odds of getting that back on that card are 100%, so easier to find than a T206 Piedmont
