Sleeping bag temperature ratings are a good, useful indicator of how warm a sleeping bag will be, but there actually a number of features that are found on sleeping bags that make them warm sleeping bags.
One of the more important features that will determine how warm a sleeping bag is, is the shape of the sleeping bag itself. With a rectangular sleeping bag, for instance, there is quite a lot of space within the sleeping bag that is never used by you. That extra does will, however, absorb your body heat. So that valuable body heat ends up being used for all this space within the sleeping bag that you will never use.
Another thing that will definitely help to keep your sleeping bag warm is a draft collar. This is a tube of material that wraps around your neck, and helps to trap warm air inside the sleeping bag.