The 500k figure is probably closer to top 1% than you'd think.   


  
Imagine a person who has a steady income of 250k but has 1M capital gains from investments every few years. They'd fluctuate from top 2% to top 1% fairly regularly but if you average it out it'd be around 500k.   


The problem with threshold statistics is that it fails to take into account income variability. Average income over 10 years would likely be a better measure.