I'm not sure but it may have to do with spline... the longer the arrow the less "stiff" its going to be, and with the high poundage of a crossbow it may have a big impact, however the longer it is the more stable it is, so I have no idea...

I've wondered about different crossbow arrow lengths myself, in fact when I finally get a good "main" setup figured out so I can relax a little I plan on experimenting some with different arrow lengths/sizes just tosee what I come up with