Wisdom card holder only one sided?

Ok, this might be a really dumb question and this isn't a big deal at all, but is the wisdom card holder supposed to have 18 slots? Mine only has 9. I fit two cards in each slot if I put them back to back, but just wondering if my component is different since the manual says it fits 9 on each side, or if that just means it was intended to have them back to back like I'm doing. Did anyone else sleeve all the cards and then realize you have to take those out to be able to fit them in the holder? Woot, extra loose sleeves put back in the collection to use on other games I guess!