It is not all that uncommon to have users judge the minimum comfort limit of an identical sleeping bag up to 20 degrees apart. Testing instruments such as the copper mannequin provide clo data using a very controlled environment. The mannequin is a fixed size and does not roll. Gary Schaezlein, owner of Western Mountain Sports, manufacturers of Western Mountaineering down sleeping bags agrees with Williams wholeheartedly. The bottom line with a bag is it must keep you warm.
Moonstone also reports that they overfill their bags to ensure warmth.
Wise field advise that I have heard dispensed over the years is that you can always unzip a bag if you are too warm, but you're going to have a problem trying to add more down or insulation if the bag's too cold. One company, when asked how they were rating their bags, went so far as to state that they had their bags tested by the copper mannequin a number of years ago, and simply extrapolated the information into their current line. Of course, those companies who choose to be more optimistic with their temperature ratings often come out ahead when you only catalogue-compare price, stuff size, temperature rating and weight. Because of all the confusion, a number of heavy hitters in the industry, including Cascade Designs, Sierra Designs, REI, Marmot and 3M insulations, have banded together to attempt to hammer out an unbiased, meaningful, understandable and standardized rating system. Don't expect anything to happen in the next year however, since not everyone in the industry agrees that standardized temperature ratings is a good or even achievable idea.