I personally think the Versalite rating is a tad optimistic. When I was researching bags several years ago the two bags I was considering were the Apache SMF and the Versalite. Interesting enough they were both rated at 15* at that time. I'm not sure why the new Versalite is now rated 5* warmer. I beleive the fill weight is identical, and the only difference is the fabric. It was 1.2 oz fabric previously and is now .9 oz fabric. It seems to me, the older version with the thicker fabric would be warmer than the newer thinner fabric. It is possible that the thinner fabric lofts better, but it certainly would not be as wind resistant. Overall WM makes a very fine bag, and I find the temp ratings to be spot on. I don't know if this helps or just adds to the confusion?