I think you're confusing median with the mean. Mean is average, so rich people at the top can bring up the average rate. Median is just taking the middle, so there will be an equal number of people above and below.
Also, the super-rich have been evading taxes way less than they used to because the top rates on the type of income they earned has been dropped low enough where it makes sense to invest again.
"The increase in reported dividends, interest income and capital gains after 2003 largely reflects reduced incentives for easy tax avoidance strategies — hang onto appreciated stock unless you have offsetting losses; avoid dividends in taxable accounts; hold more tax-exempt bonds. What has been widely misinterpreted as an increase in top incomes (and wrongly attributed to big salaries and bonuses) was largely a predictable response to reduced tax incentives to minimize reported income."
Also...
"Table 2 shows that a much larger volume of capital gains were realized in taxable accounts (rather than being unrealized or reinvested within tax-deferred or tax-exempt accounts) when the capital gains tax was 15-20% than when the capital gains tax was 28% or more. Realized capital gains only amounted to 2.5% of GDP from 1987 to 1996 when the capital gains tax was 28%, so they accounted for only 6.9% of individual tax revenues and 17.7% of the income reported by the top 1 percent. Realized gains doubled as a share of GDP from 2003 to 2007, and accounted for 9% of individual tax revenue and 28.1% of the income reported by the top 1 percent."
The thing is with these billionaires that make their money off investing is that they understand taxation very well and know how to move their money around to give the smallest amount to the government. Increasing the tax rate for dividends and capital gains too high just makes that an unviable option for ultra-rich to multiply wealth, so they move it elsewhere and no additional tax revenue is raised.