I think this is actually a better question for /r/AskScience. And in fact, this thread has a very good answer for you and the authors of one of the studies mentioned there have done an AMA that you'd probably be quite interested in, too.

edit: Here's the conclusion that tends to surprise folks the most, and shows that there's really nothing special about any of the examples in your link:

you don't have to go very far back in time to reach a point at which all individuals who have any descendants still alive are actually ancestors of everyone currently alive today

In other words, we're all probably descendants of Cheops, but we're also all descendants of every nameless peasant with any surviving descendants.