I can't figure it out. Some items are cheaper the more you get, and some are more expensive! Even by those which are officially "smart cart" items
I don't know why it's like that but thankfully it helps me cap how much stuff I end up buying.
My guess is it might have to do with current inventory of the cheapest retailer. Or it could be there's an incentive to add an item from the same retailer to your cart, but not much incentive to keep on adding the same item. I usually find that 1 or 2 of each item is the cheapest. I'll sometimes buy 1 of each size for the best price.
Jet needs to continue tweaking the smart cart but hopefully they don't ruin prices with that.