Eleven means one left. In Old English it was endleofan, from a Germanic ainlif: one, plus a piece of a root meaning to remain, so one left over once ten is done; twelve is two left, made the same way. Nobody wrote down why ten was the place to stop, but most people carry the likeliest reason at the ends of their arms. By thirteen the language has steadied itself and just says three-and-ten, like someone who ran out of fingers, paused, and decided to keep proper count from there. Eleven and twelve are the pause: two numbers English still says the way you would while looking at your hands.