2 stacked droppers do not always send correctly

When sending an item through 2 hoppers and then into a dropper pointed up through another dropper with a chest on top (see attached screenshot), the item will always end up in one or the other, depending on how it is made.

This will always happen one way or the other. I built about 5 different setups exactly alike, and 2 of them always had the passed item stop in the top dropper, and the other times they always ended up in the chest.