Since he won't release his tax returns, and does admit they'd be politically damaging, all we can do is guess.
This is one in a series of speculative diaries wondering what could possibly be worth the price he is paying by remaining secretive.
Today's idea: he's stiffed the LDS church on his tithing by tithing on his (ridiculously understated via tax avoidance) taxable income rather than the income that most rational people would say is "how much he made last year".
Remember that Harry Reid is out there saying that Romney paid (basically) no taxes for 10 or 12 years. Does that also mean that he donated basically no tithe for those years? While somehow amassing a net worth in the hundreds of millions of dollars?
Can you imagine the look on Karl Rove's face when they call UTAH for Obama?
Other diaries in the series.
Nothing, he's just pathologically secretive
Hiding stuff from Ann Romney
Rich people write-offs
Olympics corruptions
Medical expenses
Political contributions
Foreign investments
Church contributions exceed taxes