Overlaying Items Sneak Past Filtered Hoppers

expected results:
If two different items in the same exact spot pass over a hopper with a "filter" the item that the filter should accept will not pass over the hopper and be accepted into the hopper.

real results:
If two different items in the same exact spot pass over a hopper with a "filter" the item that the filter should accept can pass over the hopper and is not accepted into the hopper.

In this setup there is a hopper with each spot full of one orange wool. There is a water stream to push items past it. Above this stream are two perpendicular streams that push items into a corner. I throw a stack of white wool in the stream followed by a stack of orange wool. I break the block allowing the two different stacks to meet the bottom hopper. The two stacks both pass without the hopper taking any wool.