Not my video but https://www.youtube.com/watch?v=VGvFBqfVjBo show it off perfectly. Items are simply being "lost" when trading with villagers. For instance, the armorer has the 4 Iron for 1 Emerald trade which can be traded a max of 4-6 times. A total of 16-24 iron should be consumed but when a full stack is placed most of the stack after using that trade 4-6 times just disappears.