You are wrong. Longbows were one material bows and their maximum launch speed was very limited due the low maximum spring speed of the wood. Composite bows achieved much higher velocities (with light arrows). The advantage of the long bow, apart from being rather robust in wet weather, was that they could shoot heavy war arrows near their speed limit(about 60m/s I think) when they had a high poundage. But when decreasing the weight of the projectiles the launch speed stopped increasing at some point because of the limited maximum spring speed of the bow.
This upper limit was higher for multi material composite bows or steel bows(crossbow). They could achieve higher projectile speeds at the same draw weight with lowering the arrow/bolt weight. They ultimately run into a wall too at some point.
In cRPG we just have to image that the projectile weight is directly linked to the launcher because of the limitations of the Warband engine. Unfortunatly we can't have different arrow types lead to different projectile speeds or disallow arrows for some bows afaik.
