Hi! As the refined equipment gets higher level, the chance for a "Refine" to immediately upgrade refinement from one level to another is lower. This means that the potential chance for a refinement rune to refine a equipment gets harder as the equipment gets higher level. This is why players might find that at the higher refinements, they sometimes reach to the near maximum of "Wish" before having a successful upgrade. There is always a possibility of the refinement at a higher level to upgrade at low cost, but it is rarer. I usually recommend to have the required Refine Rune on hand just in case (if you are looking to maximize the Refinement event and Wish Credits.