A dev posted here back in the day about the range variables, though I don't know where to find it. I think he said that the Range variable determines how far from the unit the bow-and-arrow icon switches from a red X to a green arrow, and also marked the target range. However, it was the Velocity variable that affected shot power, and thus range too.

So, if the Range variable is too high and the Velocity variable too low, your archers will think they can shoot a mile but their arrows will only travel half that distance. If your Velocity is too high and your Range is too short, you just increase the speed and lower the trajectory of the projectile.

So X-bows and Arbs have very high Velocity but short range, which is why their projectiles move so quickly AND have a relatively flat trajectory. This as opposed to Longbows who have a longer Range but relatively less (but still enough) Velocity, which results in an arched shot. Also, Longbows, as opposed to X-bows, have an ability called Try-High or something which allows the computer to try for a higher-angle shot if a lower-angle one is blocked, like by a wall or something.