It's saying the same thing:
> GAO found that, as income increased, IRS recommended more taxes per audit, on average, but generally closed fewer audits.
They close fewer audits because the audits require more resources (and partially because there are fewer of those taxpayers). Meanwhile the other effect also shows up clearly in their numbers: Average additional tax for the $500k-1M group (midpoint $750k) was ~$20k or 2.67%, for $1M-5M (midpoint 3M) was ~$40k or 1.33%, for $5M-10M (midpoint 7.5M) was ~$86k or 1.15%. Percentage for the top group is unknown because that bracket is open-ended, but $360k for the group that contains billionaires is likely to continue the trend.
They get less as a percentage from the people with better tax lawyers. It's more in absolute dollars because by definition they have more money, but since it also costs more to audit them it's not necessarily worth it.
Notice also that these are heuristics and the actual numbers change with the number of audits. There are going to be some people who are obviously cheating on their taxes, e.g. someone else filed a 1099 or W-2 for paying them and then they filed a return that didn't include the income. The IRS is going to put them at the front of the list to be audited and then that audit is going to show a positive recovery for the IRS.
The more audits you do, the lower the recovery rate gets, because you started with the ones with huge obvious problems and doing more audits means auditing people who are more likely to be innocent.
This is also why the "bang for the buck" calculation is pretty meaningless. Suppose there is one person you can audit for $2000 and then recover a million dollars (+$998,000), and then an unlimited number of people you could audit for $1000 and recover an average of $800 (-$200). If you do one audit of the one who is obviously cheating, you've recovered 500 times more than you put in! Clearly you should keep going. If you audit 100 people, you've spent $101,000 and recovered $1.08M, that's still almost ten times more than you spent! Clearly you should keep going? Except that only one of them was net positive and the additional audits have actually reduced your net gain from $998k to $978k while auditing a significant number of innocent people.
What matters is the incremental returns from additional audits, not the average returns from existing audits. And taking into account as a cost the burden of auditing innocent people.