We're working out what we think - we'll add our view here!
Mountain Equipment says:
For ultra-light backpacking and alpinism it is difficult to conceive a bag so reliably warm being any lighter. Perfect for high mountain use during the summer and at lower altitudes in Spring and Autumn.
*Mountain Equipment use their own 'Good Nights Sleep Guaranteed' rating which they believe is a more accurate number for some sleeping bags and is based not just on the EN test results but on lab testing, their own use, thermal imaging and cold chamber work, along with a lot of data analysis.