How can the derivatives market be worth more than the world's total financial assets?

From Slate:

Iowa Sen. Tom Harkin issued a call on Tuesday for regulation of the “over the counter” derivatives market, which has an estimated size of about $596 trillion. By contrast, the value of the world’s financial assetsincluding all stock, bonds, and bank depositswas pegged at $167 trillion last year by McKinsey. How can the derivatives market be larger than the entire world’s financial wealth? [link]