Bows and crossbows can have variable pull in order to take advantage of Strength. A normal bow is made for a Strength score in the 4-7 range. For Strength scores of 8 or more, a specially made bow can do more damage, and conversely a bow made for lesser Strength would do less damage.
To calculate the damage of a bow made for an unusually high or low Strength score, subtract one from the damage from the base table and add 1 for every four points of Strength score, dropping fractions.
Using a bow that is too strong for you is normally impossible; using a bow that is too weak simply means you do less damage than you could.
It takes great craftsmanship to make a strong bow, so these cost an extra Item schtick.