Hard to say without knowing the supplier's volume, the nature of the products, etc., but it's entirely possible given the nature of Amazon's warehouses that those "almost identical items" could be scattered all over.

They could have arrived from the supplier in small batches (or even one-by-one) over time if it's a low-volume sort of thing. Amazon might intentionally spread them throughout the fulfillment center in some situations to expedite packing and reduce the distances their robots have to travel.

They might have just needed twenty boxes to finish the tetris puzzle to send a truck out the door.