Sleeping Bag temperature ratings are very subjective. Different people sleep "warmer" at different temperatures, or are cold if you prefer, at different temperatures. There are many factors that go into whether the Sleeping Bag you're using will keep you warm. Your overall health, your immediate health, what you're wearing as "night clothes" inside the Sleeping Bag, and especially what you're using for a Sleeping Bag pad. A Sleeping Bag temperature rating of -20 may not keep you "warm" at +20 degrees if you don't have the appropriate protection from ground cold. Conversely, if you are sleeping in insulated underwear and a parka, A Sleeping Bags with a +20 degree Sleeping Bag Rating may keep you comfortably warm at -20 below.
You should use Sleeping Bag temperature ratings as a guide ONLY. When testing cold weather Sleeping Bags, our testers (both men and women) sleep in expedition weight long underwear and polar fleece socks on camp cots. It's seldom possible to test a Sleeping Bag under exact temperatures, but our tests indicate if you're not in good health, or you have difficulty staying warm while sleeping, you should choose a Sleeping Bag with a Rating of approximately 20 degrees less than the Temperatures you expect to encounter. IE: You expect to see temps of zero, look for a Sleeping Bag with a temperature rating of -20.
As an aside, we don't recommend the use of down Sleeping Bags for survival situations except in extreme arctic conditions. While down is a superior insulator, synthetic insulation is not far behind. Down "clumps" when wet, and if (and when) it gets dirty, the insulating properties of down are severely reduced. Synthetic insulation will continue to insulate when both wet and dirty, is considerably tougher, and is much easier to clean and care for.