Actually, here's a question: He supposedly did not claim the full $4 million dollars in charity on his tax returns, instead claiming only $2.4 million dollars. If they're going off his tax returns, and he didn't claim the donations on his tax returns, what exactly are they operating off of? His word that he donated in such a way? The tax return itself can be investigated by the IRS, but would they have a reason to audit donations that he isn't claiming on his taxes?
I'm not saying he did, but if he took $4million, donated $2.4million of it and secretly deposited the rest in an account in the Caymans, what would stop him from claiming that he donated it all? The Caymans don't give out account information to anyone (the reason why it is used as a tax shelter), so Romney would appear to not have the money. Only catching the financial transfer, an audit of his firm, or perhaps an audit of the charity he claims to have donated the money to would reveal the truth.
But the kicker to that little theory? Even if the very unlikely happened and Romney was caught, he still wouldn't get into trouble, because while he would have lied about what he did with the money, he paid taxes on it anyways. No harm, no foul in the eyes of the IRS.
Again, only a theory, but I'd cock an eyebrow at anyone who would choose to make tax deductible donations, then choose to not declare some of them just so he could pay taxes. Add in the fact that Romney has commented favorably upon plans to eliminate taxes on capital gains, which as he himself said "Under that plan, I'd have paid no taxes in the last two years." and I find the tax return even more questionable. The mastermind of said plan, Paul Ryan, is now his running mate, which throws an extra shadow of doubt over his motives. Top it all off with the fact that he refused to release any other tax returns, not even a single extra year to show that such a thing was normal behavior outside of an election year, and that he can at any time after losing the election correct his "mistake" by filing to correct his taxes, and I think this is likely bullshit. Artfully timed and woven bullshit, but bullshit nonetheless.