Item durability does not update properly in the inventory. Changes in the item's quality can only be seen once it has been put down on the ground. The items in question would appear in their original state for a long time while they're equipped - as you set them down on the ground the item would properly update to it's true state.

The most common example is having the quality of your jacket worsen after being hit by zombies. If you were originally wearing a Pristine Jacket - enduring twenty zombie attacks would not seem to affect the Jacket, it would remain Pristine. Only after you've set it on the ground do you see that it's actually ruined.