Understanding EN Temperature Ratings is actually a pretty good Blog post by someone at Thermarest that explains it. There are similar articles by REI and others if you care to Google.
And, I think different companies use different numbers. For example:
the Kelty Women’s Cosmic Down 20 bag has an EN Lower Limit Rating of 13 and an EN Comfort Rating of 25.
The REI Women’s Serrana 20 has an EN Lower Limit of 7, and an EN Comfort Rating of 20.
The REI Women’s Magma 17 has an EN Lower Limit of 3, and an EN Comfort Rating of 17
How about men’s bags?
The Men’s Kelty Cosmic Down 20 has an EN Lower Limit Rating of 19 and an EN Comfort Rating of 30
The REI Men’s Igneo 17 has an EN Lower Limit of 17, and an EN Comfort Rating of 28
The REI Men’s Radiant 19 has an EN Lower Limit of 19 and an EN Comfort Rating of 32
So: REI uses the EN Lower Limit rating to rate their men’s bags and the Comfort Rating to rate their women’s bags. Kelty seems to fudge the numbers. They are NOT using the Comfort rating for their women’s bags, but something lower than that, which may or may not be somewhat optimistic on their part.
As the article at Thermarest says, your mileage may vary, based on your body mass, ambient weather conditions (i.e. potential convectional losses), dampness of your base layer clothing, whether you’ve had anything to eat right before going to bed, etc.
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