@freemo $10m and up is a huge range though. Wonder how distributed a further breakdown would show that single average number to be. That seems to be the IRS's cutoff of income bracketing as well, so probably no way to expand it more.
The question is why there is a gap and then apparent climb in percentage, and I imagine it's a combination of cost of living needs vs. donating as well the definite advantage of giving away a maximum tax deductible amount when one has it to give to reduce tax burden overall.