The problem is there's an assumption that the government data is right, and disproving their data as a taxpayer would be tough. (Fighting city hall is tough)
In the current voluntary compliance regime, a very small number of returns receive audit treatment of any kind.
A few years back I miscalculated my self-employment tax. Didn't get audited, just got a letter saying "You miscalculated and overpaid", followed by a check for the $100 or so.
The government can disagree with your return based on their data without auditing. A couple of years ago, the government told me they thought I'd under-reported my taxable income from a couple of years earlier (I had, because a place I'd worked had failed to give me a 1099). I wasn't required to undergo an audit; they just said they thought I owed this much money and I should either pay it or explain why I didn't actually owe it.
In the current voluntary compliance regime, a very small number of returns receive audit treatment of any kind.