Killing it once means you have clear experience, but it does not mean you have mastery experience. I only said that having clear experience means you're very likely going to do better than someone who did not, but I never said clearing it once means you're going to flawlessly kill it over and over again. When I build parties to farm, you need significantly more than simple clear experience for that very reason.

Also, posting small anecdotes doesn't counter my argument since I'm talking in the general sense.