Simple explanation. It is the difference between "things have mixed so much that everyone is your ancestor somehow" and "on a strict matrilineal or patrilineal

Simple explanation. It is the difference between "things have mixed so much that everyone is your ancestor somehow" and "on a strict matrilineal or patrilineal descent, everyone winds up at the same thing". In the first the path goes through both fathers and mothers. This is easier to achieve, and so you get to it more recently. The second is harder and so happened longer ago.