For years, mortgage giant Fannie Mae has produced smoothly growing earnings. And for years, observers have wondered how Fannie could manage its inherently risky portfolio without a whiff of volatility.... The magnitude of Fannie's machinations is stunning, and in two key areas in particular they deserve to be better understood. By improperly delaying the recognition of income, it created a cookie jar of reserves.... Lower rates caused a lot of mortgage holders to prepay their existing home mortgages. And Fannie was suddenly facing an estimated expense of $400 million. Well, in its wisdom, Fannie decided to recognize only $200 million,...