Several very rich families are just that, rich families.

That makes attributing large amounts of wealth for specific individuals very difficult especially when money is tied up in actual assets and private companies (Rothschilds).

Most of the billionaires on the Forbes 400 have a lot of their wealth tied up in equity distributed by a public company. Determining a billionaire by that way is as easy as getting the SEC filings and doing some simple calculations. 

When wealth is tied up in companies which don't have a share price or break down of equity, you get thrown into a situation where you have to 'guess'. Forbes isn't in the guessing game most of the time for the Forbes 400.

Another thing to realize is that with families that are passing down money through the lineage there is a lot of dilution in the form of:

Higher capital gains taxes (especially compared to the past)Estate and inheritance taxesPossibly inflationary issues

So overall don't think those people aren't as wealthy as they seem. They just don't have nearly as transparent wealth as most on the list. Sometimes calculating wealth can be like trying to see the bottom of a muddy puddle. Too murky to meddle with.