One of the core problems with the GINI index is that it measures disparity without any deeper analysis ... What this means is that the GINI index would see an economy that is shrinking at 5% per year where the top 10% is shrinking at 10% per year as being a better outcome than an economy that is growing at 5% per year where the top 10% is growing by 10% per year. If you allow this situation to continue for a couple decades the poor people in the "Unfair" economy will be better off than everyone in the "Fair" economy.
There are a couple of factors which have lead to the increasing disparity between the wealthy in the United States and the poor in the United States even though the standard of living in the United States has dramatically improved for almost everyone. Globalization has meant that companies have grown so large, and so regionally disproportionately (management in western countries, labour in China/India) that the jobs that would have represented the top 0.01% of earners in the economy of the United States 100 years ago now represent 1% of the earners in the economy today. On top of that a very luck entrepreneur today can make a small amount of money ($1) from several billion people at low costs and just make an unbelievable amount of money; and we see this happen every year as the next "Youtube" or "Facebook" is produced by a handful of people.
In contrast the productivity, responsibility and profitability of the labourers in the bottom 50% of the economy has really not changed dramatically over the past 25 to 50 years.
