They don't ignore much larger sums; larger sums are much harder to automate detection of. So with the limited amount of manpower they have, far fewer are identified.

They can send a letter asking for documentation for everything the automated system flags. The automated system detects "simple" cases at a far higher confidence ratio, which tends to be lower income. Once you get upper income, with all manner of complicated itemized deductions, the confidence of the automated system is much lower, and the effort to ask for appropriate documentation, and verify it, is much higher, for a much lower expectation of return (since, again, low confidence from the system).