Freddie Mac released its weekly update on national mortgage rates on Thursday morning, and the results this time were pretty curious.

Both 30-year fixed-rate mortgages (FRMs) and 15-year FRMs got cheaper during the past seven days, with 30-year FRMs falling four basis points, to 4.29% interest, and 15-year FRMs slipping one single b.p. to 3.38%. One year ago, 30-year FRMs cost 3.35%, and 15 years cost 2.56%.

On the other hand, 5/1 adjustable-rate mortgages (ARMs) and 1-year ARMs both appreciated in price. The 5/1 ARMs gained two basis points, rising to 3.05%, while 1-year ARMs rose a single b.p., to 2.45%. A year ago, both of these rates were 2.56% -- the same price as a 15-year FRM.

Freddie Mac vice president and chief economist Frank Nothaft noted that U.S. Q1 GDP estimates "fell well short of market expectations," which may help to explain the weakness in longer-term mortgage rates. Working at cross purposes to this trend, however, were new data showing a turnaround (upwards) in the rate of pending home sales for March after eight months of declines, and a tidy 12.9% rise in the S&P/Case-Shiller 20-city composite house-price index for February.