What makes the 'Twenty-Five Wealthiest People in History" chart open to argument? Why does it still appear that Mansa Musa was one of the richest people ever?

its to argue about because people like to argue about things like this?

Explanation:   At an estimated value of $400 billion, Mansa Musa I of Mali is the richest person who ever lived. Born in 1280, Mansa Musa was ruler of the Malian empire, and acquired the most part of his wealth from the production and trade of salt and gold; more than half of the world's supply at the time in fact.