There's one statistic that I didn't see but I'm now really curious about: what percentage of audits discover irregularities, per income level? If lower-income people are more likely than average to cheat on their taxes, then it makes sense to audit them more, but if they don't, then this is indeed totally unfair.
"The rate at which taxpayers understate their tax liability increases monotonically with income and average adjustments are highest in the highest income decile" [1].