I was told that by one of their reps at the outdoor show a couple or 3 years ago when I was discussing sleeping bags with them. I've since seen it on the web but I can't find where I saw it.
I should have added the 24 yo is also about 11.5 stone so if you're older or female you need to look to a slightly higher rating.
They should be rating the bags using the correct standard...
http://en.wikipedia.org/wiki/EN_13537
However it seems it's a option for manufacturers to use... I wonder if they do use the standard as they don't mention I any where on their website. Also it doesn't apply to Military sleeping bags....