AFAIK gear won't respond when it is maxed out. However, if you kill the same mob 2 times, there isn't any guarantee that the next time you kill it, the item you are looking for won't be on it.

Without seeing the codebase, I have to assume two things control normally (I believe there are some tweaks for barrier wands and the like) limitted gear being maxed out. That being, number of items in circulation and number of active p-files. Either of those factors could change at any point in the game.