[joetapley]

Shirt
Two aspects;
1. Given two arrows which are both tuneable the weaker arrow is likely to have some combination of lighter/thinner shaft/higher FOC so it will be aerodynamically a better performer than a stiffer shaft. You will generally get better groups with a weaker shaft.

2. For bare shaft tuning the fletched arrow will behave on the bow dynamically stiffer than than a bareshaft one (physical weight at the back of the arrow of the fletchings and the 'added mass' of the fletching drag). So if the setup is perfectly tuned for the fletched arrow, with bareshaft tuning the bareshaft arrow will be 'weak' and hit to the right (RH archer) of the fletched arrow.

The reason it's often recommended to have a stiff arrow or for the bareshaft arrow hitting to the left of the fletched arrow is I think a hangover from the wooden bow era where you didn't have much of a bow window so you sacrified tuning for arrow clearance. I think longbow archers still use the bareshaft arrow low and left approach as this gives good clearance round the bow and over the bowhand.