Ratings are a general comfort, or safety rating, and not a guaranteed comfort rating.

Why can't the manufacturer determine comfort ratings?

Comfort is relative to the individual. One person might be cold at 60 degrees, another at 65 degrees, and another at 70 degrees. Safety is a constant. Water always freezes at 32 degrees. Therefore many bags have a safety rating.

Manufacturers may try their best to rate a bag for comfort, but it isn't a sure thing. It is almost impossible to determine a person's personal comfort level. Here are a couple of good examples. If a person from Phoenix, Arizona visits northern Idaho in the spring, they will say it is cold. However, a person from northern Idaho will be just fine. A person from Miami, Florida is going to have a lower tolerance for the cold than somebody from Anchorage, Alaska.

Another example, my wife lost a lot of weight. She used to be warm all the time, now she is always chilly and wears a sweater most of the time. So I guess we could say she has had two different comfort levels. How would a manufacture know which one to use? I have sold thousands of sleeping bags and have had a lot of good feedback and many repeat customers, but every once in a while I get a customer that was cold, usually from a warm climate.

A few suggestions:

1 - I would recommend referring to your own experiences and if you tend to sleep cold, get a bag that is rated 10-15 degrees colder than you anticipate using it at. But with that, the weight will increase.

2 - Use a liner or sleep in a pair of thermals. It is easier to pack a pair of thermals that weigh 1 lb. or so than a bag that weighs 4 lbs. more.