durability mystery

Maybe to go into further detail regarding Schnorki´s answer (which is ofc 100% correct)
An item is on drop "checked" if its unique, rare or magic, otherwise it´s normal. This "check" is just a pre-roll (probability to be a unique, rare, magic) basically. This is done in order, e.g. first a check is done if it´s unqiue, if it´s not, next roll is done to check if it´s rare etc, etc.
Now, for some items there is a probability to be a unqiue, but no unique of that item type exist. In this case, if it´s rolled to be a unique it gets automatically a rare but with increased durability (see Schnorki´s calculation). This is called a failed unique. Btw. this also applies to item types which have a unique, but the unique item(s) already dropped in the game, so there is no unique available anymore. Each unique can only drop once in a game.
And, because D2 is not just a simple mechanics game, another way to retrieve failed uniques is if the monster which drops it, does not have a treasure class high enough to drop the unique equivalent.
Hopefully this sheds some light into the failed unqiue topic. More infos can sure be found, most likely even explained easier to understand, in other topic/articles .