okay so second richest man? third? tenth? how does that change the calculus?

The largest entirely private fortune.

Individuals that are now or have been heads of state may have had opportunities to seize or otherwise employ state-owned property for their personal enrichment. Dictators over petroleum exporting nations are all likely richer than Bezos, in ways that we could never expect Forbes magazine to be able to verify, or even begin to investigate.

People whose wealth came mostly from publicly-traded stocks and real estate in nations with public records of title transfers can be more easily tracked and verified.