Originally, because that was what I was told at my pro shop, but being the way I am, I tried orienting the cock fletch in just about every direction. I found that I had fletching clearance issues whenever any one of the feathers were pointed down within about a 20° range from being perfectly vertical. I was not using my limb driver for these test. I used the QAD Ultra HD. This caused me to conclude that by orienting the cock fletch up, there would be more room for the fletchings to clear the arm. Make sense?