Getting all caught up in the details gets confusing, needlessly. It's just how much loft the bag has, basically. Everything else is details. That's how every good article about warmth discusses temp, just by how much loft there is, and that's really all you need because that's an actual fact.
You may be younger so you don't remember when companies like Northface and Sierra Designs were real companies run by real backpackers and climbers, their gear sewn right here, in the SF Bay Area, back then they were great. And honest. I'm comparing bags and loft, it's not related to my age, I can see with my eyes that bags sold as 40 aren't, 20 most certainly aren't. My body merely confirmed this fact to me at night. And I can stick a new wm 20 degree bag next to the old nf 20 degree, and the new nf 20 degree, and it's self evident that the new one is falsely listed as 20, it's lucky to be 35. Not related to age, just related to companies lying and getting greedy.
I can show you a picture of two twenty degree northface bags, one from mid 90s I think, one from 2011. Half the loft in the new one, and the old one also had that warm fuzzy bag liner built in (so heavy, but oh soo warm and comfortable...), good for at least 5 degrees by itself, maybe more.
So really just pay attention to how much loft the stuff has, and don't worry too much about the complex math and all the testing. I have hopes that Kelty and REI are still somewhat honest in their practices, but generally optimism like that isn't warranted, but I do have hopes.
I bought a 50 degree lafuma bag which in a sense has no loft since the fill is so loose, that was before I realized how seriously things had changed here in terms of what was considered acceptable in advertising. Silly me.
It's a drag that you have to now learn a fair amount to actually get what you need, but that appears to be the case in almost every area of outdoor gear now. Maybe not backpacks or sleeping pads, not sure there. But he's already got a pack so he's set there.
However, rereading your posting, I realize I must have failed to communicate properly, both northface and lafuma sold their bags but did not list the actual comfort rating, they listed the mid rating, where you are kind of miserable but will survive the night ok. Western Mountaineering lists the actual comfort rating. That's why nf and lafuma are frauds and why the good companies like wm, ff, and a few others, aren't. And that's, again, about loft. And those good companies tell you precisely how much loft you are getting with each bag.
Basically, to put it into simple terms, its like a car company using its highway mileage for city mileage and pretending they didn't do that. Ie, it's fraud. Legal, somehow, no idea how, but legal nonetheless.
If you have access to the actual EN rating range chart, and if you understand what it means, and if you totally ignore the advertized rating, the top end rating is going to be reasonably accurate, only it's not related sometimes to what the company claims for the bag. Knowing this is a good way to avoid expensive mistakes. I wasn't aware of this change, coming from an old school where the numbers reflected reality, so I trusted those numbers, which was a mistake, a fairly expensive one.