Yeah. The WoW system knows if you've unlocked it, which means "had it in your possession at some point". When you do that with a lot of items, you can track that with 1 bit. So you can track unlocks on a huge number of items in a small amount of storage space. That doesn't let you track dye or anything like that, but for unlock purposes it doesn't matter (you'd need to dye it as part of the glamour process instead, in that case). AFAIK the armoire works like that right now, which is why only certain items can go in it.
The glamour closet is working more like inventory, in which case you have to store which items are in it. That require at a minimum some kind of item key to indicate what item is in that slot, and possibly also a slot number if it's not a fixed data structure like an array. In this case you don't need a quantity because you can't have more than one of a given item in the slot. But an item key is a bigger value to store, so you need more storage space to do it if you want to store a large number of things in there.
Without knowing what their backend looks like it's hard to know why they chose to go that way considering the limitations it creates. Perhaps they didn't have the budget to sort out how to do a dye on a glamour of an item without doing it on the item itself, which would be a problem in a WoW system and likely doesn't exist in this one (assuming it works the old way where you'd dye the item itself and then cast the glamour).
I bet it'd be really fascinating if we could ever get a technical talk from someone on the team about why this stuff works the way it does.