The screenshot tells the story. If you put items into the top chest, the result will be that 50% of the items are in the bottom leftmost hopper and 50% in the rightmost. The ones in the middle will get no items at all.

There is a workaround for this by creating a clock circuit to active the second level of hoppers only for a certain interval (so that the second row of chests have more than one item to pull) but that's not such a nice solution.