When making a horizontal line of hoppers you would expect items traveling through to stop in each hopper's inventory for at least long enough to be visible, but some hopper's in the line will actually always have an empty inventory, even though items pass through. I believe the hoppers are passing their items they received that tick on to the next hopper in the same tick or the immediate one after. This can extend to 2-3 hoppers in a row. This is a bug because it affects hopper sorting machines and pulling specific items out of hopper lines.