The first uses yellow inserts to place content onto the blue belt, and then yellow inserts to remove the content, where the yellow inserts are restricted to collecting only 1 item:

If you load the save, then place down the electric supply, you'll end up with this result after 100 items are placed on each belt

For the bottom belt, all 100 items are collected, but for the top belt only 33 items are collected, with 67 making their way through to the end container.
This is then flipped by moving the items to the other side of the belt:

The result is a mirror of the first

While the mirroring is a good thing in a way, the question remains? Is it intentional or accidental that the inserts have more trouble collecting from one side of the belt than the other?