This from an old Five on Friday (which I miss):
"I'm a Legendary Smith - why do I lose durability points so often when I try to repair things?"
"Because there's a pretty high base chance to lose a point of durability" seems to be the simple answer. The formula is as follows:
Chance to lose Durability = ((40 + Max Durability) - Current Durability) - (repairSkill / 100) (where repair skill is Test Center-style, i.e. GM = 1000, not 100.0)
Example: Blacksmithing of 1100 (110.0), 50 current and max durability, yields a 29% chance that max durability will be reduced by 1. If an item has taken more than 71 points of damage, the chance for it to lose a point of durability reaches 100% even for a Legendary smith. So there's a definite choice to be made - do you repair early and often, and risk failure, or late and seldom, and guarantee it?