In reply to jezb1: I only skimmed through, looking for something which wasn't there (I had a different kind of article on sleeping bags in mind upon clicking), but it's because women generally feel the cold a little bit more than men.
Good and useful article. A few years ago I got a cheap synthetic bag supposedly comfortable at minus 5. I thought this would be warm enough for Spain in March. But it was freezing. The temperature only went down to +3 at night but still the bag was not up to it.
My best bag was one I swapped with a mountaineer many years ago. It had 1400 grams of down and was a proper expedition bag. It was great. I remember sleeping at Hueco Tanks hut when the temp fell to -14C one night. Everyone else froze but I didn't even need to put my t-shirt on. That bag had so much loft even when it was empty it looked like someone was inside it.