It really helps with understanding this to look at Marmot bags - I like that they post the EN rating for each bag, makes distinguishing between one 0 degree bag and another 0 degree straightforward. They refer to the Teton as a three season bag, for example, and refer to it as a 0 degree despite the rating of 15F. the EN comfort rating for it is 17.4F. The very similar never summer is rated 0 degree, is EN rated just a few degrees warmer (comfort 15.8), weighs about 7 ozs different. Very close in pricing too. Then look at the Sawtooth, the 15F 600 fp bag - same shell materials as the other two, just 7 oz lighter than the Teton, EN comfort rating for women is 26F.

Makes one wonder how they come up with the labeled ratings, the bags are so close in materials and more broadly EN rated.