From what I've read, what Bernie posted is correct. Hill intentionally shortened his draw length to 28" due because it was hard to find a reliable supply of arrows spined heavy enough for him.

POC was "the" arrow in his day. Of course you could increase the diameter for a heavier spine, but that wouldn't have been logical--one would have had to change the machinery for the arrows, the points, the nocks, and it would have thrown them even further off center on Hill's bows, which were already 1/8" or more shy of center.

The English shot heavy bows for distance and to penetrate armour--not for pin-point accuracy. That was also the reason for the long draw.