Why millions of people are getting hit with a surprise tax bill this year

I suspect that many Americans do not actually compute their effective tax rate each year. I think this is happening because people I talk to seem to construct their beliefs about tax rates based on claims made in political rhetoric and by journalists, but then proceed to astonish me by not knowing their own realized income tax rate from the prior year when I ask them.

This article, and the linked twitter posts, suggest a great strategy for lobbying the public: increase withholding rates while claiming to lower taxes. I wonder if we will see more of this?

All I know is while my refund is smaller than I expected, when I ran the numbers my tax bill is 1% less of my gross than last year. So the withholding math from my paycheck made things more even.

TL;DR: The tax withholding calculations changed, so less people are overpaying on taxes (withholding from each paycheck is lower), and thus less people are getting refunds, or are getting much lower refunds than last year. The overall tax burden still went down for most people.