You lost some hives in a "bad winter stretch".  Bees live quite well in
northern Canada and Alaska.
I don't imagine you checked their trachea for tracheal mites with a
microscope.
The great majority of colony death in December or particularly January when
there is still plenty of honey stores in the hive is tracheal mite
infestation.  Unfortunately, because this mite is microscopic and beekeepers
can't see it, they
blame the colony death on everything from El Nino, lousy queen breeders, to
the beekeepers polka dot red underwear.

I think you better check for tracheal mites.

George Imirie