The greatest of all Khans the Great Genghis Khan was not a Pashtun. He was a mongol. His father was a small Khan who ruled parts of Mongolia. When Temudjinn became Khan he conquered half of Asia including China and all of Afghanistan. All the Pashtuns who are otherwise great warriors were dead scared of the real Khan.
So the Pathans may have been inspired by this Mongol and copied the Khan from the real great Khan.