a) People who filled in the wrong number on the spreadsheet that is taxes for whatever reason, and the audit is informing the filer that they filled it out incorrectly. I mean, really, taxes should start with the government sending me the form of what it thinks I owe and I should be making corrections to that, since the government already has this information and has done it, and that would make many of these audits go away.

b) People who misunderstood eligibility requirements and claimed deductions they weren't entitled to.

c) So I don't know how these people are counted, but there are absolutely millionaires and billionaires out there cheating on their taxes and claiming no income (e.g., the current president). It's totally plausible that they get listed in the "under 25k income" audit section despite the fact that they are in fact the uber-rich that is the intended target of the outrage.