Companies rate men's and women's sleeping bags differently based on research into how bodies lose and retain heat differently. Obviously there is a broad range of body types and physiology for people, but companies find that generally speaking women tend to run colder than men and they require insulation in slightly different locations in the sleeping bag.
This is why sometimes you'll see a women's sleeping bag and a men's sleeping bag rated the same temperature rating but the women's bag might be heavier because it has more insulation in it. Or, you'll have a women's and a men's sleeping bag with the same specs (weight) and be different temperature ratings.
The best thing is to know yourself and whether you run warm or cold and purchase a sleeping bag that best fits your needs. This Expert Advice article might be helpful as well.
I hope this helps, thanks!
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