This statement is not true for all bags and especially not true for bags with an EN 13537 rating, depending on which part of the rating you are talking about How a bag is rated is important. If a bag has an EN 13537 rating, you can get an accurate picture of the temperature range for the bag. All bags sold in the EU have to be tested.

Here is a simple explanation of the rating system-
http://en.wikipedia.org/wiki/EN_13537

Here is an example of the rating of a MacPac bag-
Weight
1.5kg
Colour
Left hand zip, Right hand zip
Fill weight
800gms
Fill power
800 loft
Temp rating
Comfort -7C / Limit -14C / Extreme -35C

As you can see, there is a 28C difference between the comfort level and the extreme level. The lowest practical level is -14C for this bag, which is about +7F. MacPac bags are high end bags; I have one I bought back in the 80's. Not this model though. Mine is rated to -5C and in my experience, that is pretty accurate.

I don't know which American bags have an EN 13537 rating since most manufacturers don't sell them in the EU and don't want to spend the money to have their bags tested for that market.

The closest WM bag to the MacPac bag I could find is the Antelope, which is rated to -15C. The WM bag is about 12-14 ounces lighter overall than the MacPac (which I attribute to the bag's material and construction, not insulation), has slightly less fill in the standard version, the same in the long and the temp rating is almost identical to the EN limit level. Based on this comparison, it appears the WM ratings are very comparable to the EN standard.
http://tinyurl.com/yar983w