Or it's just plot......I mean kakashi is playing a vital role in the story and kishi needs to keep kakashi in the picture regardless of his past restrictions. I don't think kishi is thinking week by week about making kakashi stamina/ability's more believable over the plots progression because from the writer's perspective it just wouldn't make sense. Also take into account kishi writing style. This guy leaves lots of plot holes that we are left to infer on what or why it happened so i would not be surprised if this was one of them.