It's statistical. They are not calculating everyones taxes, they are saying "well you act like a duck and quack like a duck earning this much, is what your paid/what you purchased within some confidence interval of normal?" No? audit.
Do you have actual knowledge that it's statistical, or are you just guessing? Because the IRS absolutely does have all the data they need to perfectly fill in most people's tax returns. Sure, having that data doesn't mean they've done the work to be able to do things with that data, but it's not hard to imagine they have a system to calculate everyone's taxes, given the data at hand.
