Set Completion, is a weird animal

I'm trying to complete the enchanted tales portion of woe, The main set is already completed. I have opened close to 500 woe packs since release. I was under the impression that the pack opening system tried to in some ways coordinate rares and mythics so you would get missing cards, why are the secondary sets so spartan even after set completion. Its crazy to me that even at 500 packs in I still am missing 5 rares, and only have one of a significant portion.