If most archers start shooting as soon as targets enter the range, and if their individual ranges are clustered around the average, wouldn't that result in a relatively high density of shooting when the distance to a moving group of targets coincides with their average range?

Some movies actually provide a plausible-sounding explanation for the volley fire thing. A dude in charge commands everyone to wait until the unsuspecting enemy enters the kill zone. But even in that case, I guess the archers won't wait with their bows drawn.