Hopper "pipes" do not always transfer items to hoppers placed underneath them

1) Place hoppers connected to each other horizontally (hopper "pipe").
2) Place more hoppers connected underneath the hoppers in #1, save for the very first hopper (see photo).
3) Add items into the first hopper in the pipe in #1.

Expected result would be that the first hopper in the bottom row would receive all of the items. However, intermittently, the first (and sometimes second, or third) hopper underneath is skipped.

The photo shows this outcome. The first two hoppers were skipped over and the 3rd hopper below got all of the items after items were placed in the top right hopper.

Destroying this and rebuilding it will produce different results every time. The longer the pipe, the more random it seems, but certain bottom hoppers are almost always skipped.