The discrepancy between $10.4 million net worth and $515k income (which I assume includes gains) confused me for a moment, until I realized that the $10.4 million figure likely includes at least one large non-income-producing piece of real-estate.
Obviously. But if the figure is exclusive of their primary residence their return on investments isn't especially impressive-- e.g. it's sub index fund rates even assuming they have no other income than gains. :)
I wouldn't expect 'given' year effects to be relevant, as it's averaging over a large number of people. Taxable is a better argument, since as you reach the highest marginal tax rates there is a lot of reasons to find ways to indefinitely defer paying taxes (e.g. die with it and pass it with stepped up cost basis).