The 80/20 rule (Pareto principle) is found to be applicable almost everywhere. So why shouldn't it be applicable to income?
top 20% have 80% of wealth top 20% of that 20% (4%) have 64% of wealth top 20% of that 4% (.8%) have 51% of wealth
This is inline with what we see in the real world:
http://www.theguardian.com/business/2014/oct/14/richest-1percent-half-global-wealth-credit-suisse-report