Mitchell-Yes there are a number of real world variables that can raise havoc with a rating. Those that come to mind in priority sequence are as follows:
-The standard assumes you are using a pad equal in R-Value to what the bag was tested with. In the BPL world I see people routinely using a fraction of the R value used to rate the bag to save weight.
-The standard assumes you are sleeping on your back.
-The standard assumes you are sleeping in the equivalent of a jogging outfit (.5 clo).
-The standard assumes you are out of the wind. Forced convection will significantly reduce the rating.
-The standard assumes you are not using a bivy. The dead air space between you bag and the bivy will add about .5 clo in additional warmth.
I have looked at the detailed EN13357 reports for quite a few bags. The test uses a sophisticated thermal dummy with 20 different zones. On the plus side I have been amazed to see how many the rating drops with a poorly fitted foot box, the absence of a zipper draft tube, a poorly fitting hood, etc.
The major benefit of EN13357, in my humble opinion, is that it clearly defines the test variables so you can accurately compare bags. I have attached a chart, which I created, to illustrate the point. All of the lines represent standards or recommendations from various experts. Most of them don’t mention the pad’s R-value, sleeping posture of the tester, clo value of the clothing worn, tester’s BMR, wind speed, or if a bivy was used. In other words, as you stated, “There are so many variables to the "real world" experience of warmth in a bag that a normalized "standard" seems hopelessly idealistic.”
EN13357, in my opinion, defines the majority of the variables so as to not be hopelessly idealistic.