Three private mortgage insurers say a new study shows that their proprietary automated underwriting systems approve approximately 10 percent more borrowers for loans than do the AU systems of Fannie Mae and Freddie Mac. The study, done by Mercer Management Consulting, ignited a new round of sniping in Washington. The researchers concluded that 89.5 percent of a sample of loans were approved by at least two out of three AU systems developed by private MIs,