The “Item Durability Increased” enchantment only grants the extra durability until the relevant piece of equipment is swapped off of the character, upon which it resets to the original value. (in this case 100/174 ; the 74 extra coming from the enchantment).
After rigerous testing I can confirm that it applies to ANY EQUIPMENT (weapons and armor) and that it indeed negatively affects “based on durability” enchantments such as the one on the Boarskin Reaper (my beloved <3) in the picture.
Please, I beg you, make my beloved whole again 
