I haven't done the calculations yet, but it always seemed to me that the later sectors have a better chance at rarer items than the earlier ones, and that you got more items from later sectors than earlier ones. Far Eastern Coins are found in a really early sector, and a rather late-game sector, so it's not surprising they're pretty rare. They also don't drop from the endgame sectors (17-22), only iron giant cores and toy box schemas drop in those.