Perhaps it's to do with the depth of cold at origin source,, for example the atmosphere is cold to some depth higher up, as I recall that wasn't the case the past few years. At 500hPa temperatures must be close to -40C, it probably has some bearing on the type of precipitation.
Just as a side thought... -40 degrees at 500hPa are usually good conditions for Polar Lows. (I'm not suggesting such a thing though, wrong type of setup)