Ben Smith of the Politico quotes Howard Wolfson as saying that Hillary's tax returns will be out in a day or two. When they're released, just about everyone will look at the returns as an accurate disclosure of the Clintons' income. But the Clintons' tax returns will only show income that's taxable in the United States. If the Clintons earned income outside of the United States, it's possible that they've kept some or all of that income offshore. If so, that income need not appear on their tax returns.
Let's say the Clintons earned several million dollars in, oh, I don't know, Kyrgyzstan. If they keep that income offshore, they won't need to pay U.S. income tax on that income. If they don't pay tax on that income, it won't appear on their tax returns.
It's perfectly legal and lots of individuals and corporations with substantial foreign income do it because the income tax rate in the Cayman Islands, or Nevis, or wherever is effectively zero. So you keep your foreign income offshore, invest it, and only pay tax when you domesticate it (i.e., bring it into the U.S.).
As a result, the Clintons' tax returns may need to be taken with a huge grain of salt. Whatever income they show from domestic speaking fees, book sales, etc. may not include income that is generated overseas.
My guess is that 98% of Americans know nothing about offshore accounts and no one, including the media, will think to ask about it.
The Clintons will have carefully thought out how much foreign income to declare and therefore reveal. I think we should be aware that there may be a lot more money offshore, which is generally protected by bank secrecy laws and which the Clintons will have all sorts of reasons not to disclose (e.g., they have an obligation to partners to keep it confidential, etc.).
There was a diary on this earlier today that has scrolled off. Commenters there made the point that simply having offshore accounts like the Clintons do is a political liability even if the accounts are perfectly legal.