Because inconsistencies in ignition and wide velocity spreads will result in vertical dispersion at real distance.
that's what's been explained to me. I'm sure Rick will correct me if mistaken. I'm not a match shooter.
We can also determine that with a chronograph.
My point being that if it's calm (min or no wind), any large spread in horizontal should not be ignored. If it can reasonably be explained by wind, no problem.
I've seen folks just use the vertical with no regard for 4" spread at 300yds on a calm day - I don't buy ignoring that.
