If you create a hopper sorter in this well known configuration,
http://i.imgur.com/XR2maT3.png
With 18 of an item (like bones) in the first slot of the top hopper and the remaining slots with 1 of the same item,
And if you have a row of hoppers above this feeding to many of these sorters,
Only half of the items will pass into the sorter. The rest of them will continue along the row, usually meaning they are lost.
The expected behaviour is that an item should always be sucked into the hopper below (if there is space for it) before it is pushed to the adjacent hopper (that is indicated by the direction its "spout" is pointing).