My experience is exactly the same. I normally take backish roads during the summer. We have one that loops almost all the way from my house to my office and it's one of those pseudo two laners. Big enough for two lanes, but no markings. So I 'take' the right lane and cars have no trouble or issue passing in the the left. All that changes in the winter. In the winter the "right lane" is now 3 or 4" of loose and semi packed snow. If I ride on the left lane cars can't pass.
However, the main roads have both lanes cleared. I find riding the right lane, cars for the most part have no problem passing on the left. You always get that one jerk that has to make a big production of how put out they are to have to go around you (i generally find motorists less tolerant in the winter).
The worst part of my commute is the parts between main roads at the beginning and end.