A single man can be your ancestor in more than one slot on your family tree.
Taking an average human generation time of 25 years, you would theoretically have 2^30 = roughly a billion ancestors 30 generations back, or 750 years ago. But the population of the Earth in 1300 was... maybe 400 million. (e.g. https://www.census.gov/data/tables/time-series/demo/internat... for world population estimates)
And your ancestors in 1300 aren't drawn from the entire population of the world; they're drawn from a tightly restricted subset of that.
And if you think that's ridiculous, imagine how many ancestors you'd have thirty-one generations back!