A steady decline in GSE refinances throughout 2013 coupled with faltering purchase mortgage activity during the final third of the year helped contribute to an overall dip in the volume of single-family mortgages securitized by Fannie Mae and Freddie Mac both on a month-to-month and year-end basis, according to a new Inside The GSEs analysis. Fannie and Freddie issued $55.8 billion in single-family mortgage-backed securities in December, a 4.9 percent decline from November.