I’m going to agree with Jon Chait that confusion about exactly how many people pay how much in income tax is almost certainly driven by the fact that the situation is actually confusing:
Most people make no distinction between “income tax” and taxes, period. That’s why conservatives constantly harp on how much “income tax” is paid by the rich. They know full well that the vast majority of the audience will read “income tax” as “tax.” Indeed, conservative elites themselves often fall for their own rhetorical trick, getting a piece of information about “income taxes” and then restating it as “taxes.”
Now, there’s no good reason for people to confuse income taxes and the federal excise tax on alcohol. But the difference between the income tax and the payroll taxes associated with Social Security and Medicare is quite obscure. If you do what I do and put your tax information into Turbo Tax, it just adds everything up and tells you how much money you owe the IRS or how much the IRS owes you. I just filed my taxes a week ago, and actually have no idea how much of what I paid was income tax and how much was payroll tax. There are a lot of public policy matters on which I blame the public for its ignorance, but the fact that for most people most of the tax they pay on their income isn’t technically “income tax” is a genuinely puzzling situation and most people probably just assume that the term covers all taxes that are assed on your income.