Random chance? If you are getting lots of good bonuses, there is less 'value' left over for the damage stat. With the maximum damage range of 71-100 for dropped/found items, there are fewer possible bonuses.
Unless you are at a fairly low character level, if you keep reloading you should eventually start getting the maximum damage. The higher the character level you are at (within reason) the more likely you are to get stats in the upper range for any particular piece of equipment.
