Classic example is doing a startup (and failing to take off). You have a very large nominal wealth, but you practically bankrupted yourself, because as a founder you probably spent all your money to try to raise more funds, or on sales, or on product development.

Yes, the Forbes list is different.

And yes, erikb's point about being a billionare is strange, because not many people would consider anyone a billionare just because someone funded a company with nominal value 1B$.

However, erikb is right, that rich people have cheap access to enormously powerful success multipliers. (They can very realistically - almost literally - get a cheap loan from any of their dad's neighbour, or get in touch with anyone, and so on.)