Officials at Freddie Mac and Fannie Mae say that falling purchase volume shouldn’t be taken as a sign that they are backing out of the subprime market. There’s no denying that both Freddie and Fannie have eaten a smaller slice of the subprime pie recently. The drop-off has been most acute at Freddie, which purchased just $9 billion in subprime mortgages last year, down from a hefty $18.6 million in 2000. Meanwhile, Fannie Mae bought