Fannie Mae and Freddie Mac mortgage- bond prepayments slowed last month more than some analysts projected, as a wave of buyouts of loans from the debt failed to materialize and refinancing was limited amid record low rates. The so-called constant prepayment rate for Washington-based Fannie Mae’s 30-year, fixed-rate securities fell to 15.4 from 18.1, JPMorgan Chase & Co. analysts led by Brian Ye wrote yesterday in a report. The analysts had expected the rate, which represents the share of the debt that would be retired in a year at the current pace, to decline to 16 to 17.