Standard & Poor's on Friday night slammed the U.S. economy and its political system, lowering the nation's AAA credit rating for the first time in history. Rival credit rating agency Moody's dissented, reaffirming its Aaa rating while placing the country on negative credit watch. Some analysts have been making hay of the discrepancy, particularly as a way to delegitimize S&P's decision. But a review by Fortune shows that the two agencies regularly disagree over sovereign credit ratings. When it comes to rating the 15 nations with the highest GDP, S&P and Moody's only sync up 66% of the time. For five countries, including the U.S., they do not. Three of the disputed sovereigns -- Japan, Brasil and Mexico -- differ only slightly. Japan, for example, is rated AA- by S&P and Aa2 by Moody's. A perfect match would be AA- and Aa3, or AA and Aa2. Italy is the only country other than the U.S. to have an entire category differential, with S&P rating it A+ and Moody's rating it Aa2. Both Japan and Italy currently are under review by Moody's for possible credit rating downgrades.