Now that I think about it, this is really just a symptom of a deeper problem: the "rarity" of a card is a yes-no flag which has very little connection with how rare the card *actually* is in reality (in reality, rarity values lie on a spectrum, not a black-and-white yes-no thing).
For example, Pharaoh is officially rare, and yet I think it's actually a pretty common card in practice, for the simple reason that it's very commonly won from both AI3 and also from Osiris (and it's a useful card, so you see it in arena occasionally too).
On the other hand, even though it's for sale in the bazaar, Antlion is a pretty rare card, for two reasons: first, it's nonexistent in AI3/AI5, and second, it's almost nonexistent in arena/pvp because it sucks. So, oddly, you end up seeing Pharaoh a lot more often than you see Antlion, even though the former is supposedly rare and the latter is supposedly not.
What if special spins chose cards based not on the official rare flag but rather on what is actually rare in reality? Food for thought.