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If you've ever tried to conduct a deep analysis of Bank of America (NYSE:BAC), or read through its 10-K, the annual report it files each year with the Securities & Exchange Commission, then you know better than most what Winston Churchill was getting at when he described Russia as a "riddle, wrapped in a mystery, inside an enigma." Fortunately, this is about to get slightly easier.

So long FAS 91

Bank of America is an incredibly complex organization. It has $2.2 trillion worth of assets on its balance sheet, offers many dozens of financial products and services, and oversees an additional $2.4 trillion worth of assets that are owned by clients.

To top things off, it's long been the only major bank to use an esoteric accounting rule to gauge the value of certain debt securities that are carried at fair value and held to maturity. Known as FAS 91, this has caused Bank of America's net revenue to fluctuate wildly on a quarter-to-quarter basis.

In the most recent quarter, for instance, Bank of America reported net revenue of $20.6 billion under generally accepted accounting principles (GAAP). But if you exclude the impact from FAS 91, as well as other similar adjustments, its revenue in the three months ended June 30 was $21.8 billion.

This makes a huge difference. With the FAS 91 adjustment, Bank of America's revenue fell by $1.6 billion compared to the same quarter last year. Without the adjustment, it grew by roughly $100 million.

The issue boils down, in particular, to the impact of interest rates on mortgage-backed securities. When rates rise, the value of these go up. When rates fall, MBS values do, too. But while most banks accounted for these changes in value over the life of the securities, FAS 91 required Bank of America to do so all at once in each accounting period.

The net result was that it took an additional step for investors to accurately compare Bank of America's performance in any given quarter to that of its peers. And because interest rates have been on the decline essentially ever since the financial crisis eight years ago, it means that Bank of America's revenue and earnings suffered more than most from the trend.

But this will no longer be the case. Bank of America announced at the end of last week that it has decided to do away with FAS 91. It will now treat the impact of changing interest rates on the value of its securities in the same way that other banks do.

It's a minor change that doesn't impact Bank of America's fundamentals. But it's nevertheless an adjustment that current and prospective investors in the nation's second-biggest bank by assets should welcome with open arms.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.