I Was setting up a hopper Sorting machine and i noticed that not all the blocks i was sorting went to the correct containers, Curious i repeated the sorting and found that while some blocks were being properly pulled from the sorter row other blocks were passing the ideal drop point and going to the nearest empty hopper, the materials i have found to do this were spruce wood planks and Nether Quartz, In additional tests the materials in question bypassed several hoppers and collected themselves in one far down the line.

Below is the setup i am using, From what i am told Hoppers below have priority over hoppers pushing the materials forward, i have each of the lower hoppers filled with the sorting materials.