Overall, winter was rather mild across much of Eurasia and North America, with the exception of Alaska and parts of Canada. But, that means that a lot of cold air has stayed in the polar circle, helping to freeze more of the Arctic ocean and the surrounding areas.
The latest sea ice analysis shows that during this rather mild winter in the mid-latitudes, the trapping of the colder air in the polar circle was beneficial to the growth and recovery of the polar icecap. Every winter the Arctic ocean freezes, reaching a maximum sea ice extent around March. below is the graphic from NSIDC, showing the ice growth progress and a comparison with 2012, which had the lowest sea ice extent on record in September. We can see that the freezing season began at rather low levels, lower than 2012, but the stronger than normal polar vortex has helped to keep more cold air in the polar regions, promoting ice growth. The second graphic is a temperature anomaly analysis for the polar circle, showing the colder than normal phases during winter, and especially currently, greatly aiding in the sea ice growth process.