Hello Mark. I took the time to read the papers that the article you posted referred to. Although there can be variation, the standard is designed to keep people alive. So far, I have not used a bag with the EN rating that has been what I would call under insulated.
I do consider it good practice to always wear dry thermal underwear, a beanie (hat), socks and use a thick mat in winter. In saying this, I might possibly be dressing in the same manner in which the manikins are being dressed and therefore the temperature rating is accurate for me. In summer the rating variation is not such an issue as the temperature will not generally have you hypothermic but in winter it's a different ball game.
As far as I can tell, the rating system is a big leap forward however it does need some work. Anyone with winter camping experience will do fine as they have learned not to cut it too fine with ratings.