until montbell rates their bags like all those eurobums ... id rather be conservative ... case in point being the summerlite which everyone says is very warm ... yet tests to 35F
another one is the valandre mirage which BPL said
"The warmest bags in this roundup are the Nunatak Alpinist, PHD Minim 400, Sierra Designs Nitro 30/Spark 30, and Valandre Mirage. Three of these bags are rated at 23 F (-5 C)"
yet if you look at the valandre test for that bag, it tests to 30F lower limit comfort ...
i love their marketing spiel ...
Field testers have taken the super light Mirage to 14°F (-10°C) wearing a base layer of underwear and a fleece vest. Even if these are not recommended temperature ratings (metabolism etc. should be considered), field tests have certainly underlined the extraordinary performance of the Mirage.
EN testing isnt perfect ... but its like EPA gas milleage ... its consistent and comparable ... without it marketing departments would just make up any figure they want
i actually give a lot of credit to those companies that have their products tested ... and also to ones like LL bean and Lands End that tests the rating of their clothing scientifically
ultimately do i think 1 or 2 deg matter a lot? ... nope ... worry about other things instead ... a 5F+ gap does look funny though ... and 10F .. well id consider a properly rated and tested bag
if youre young, dumb and full of ... well ... that WM summerlite will prob be a 25F bag for you anyways ;)
here's a sample of the variability in ratings before EN ratings ...