Here’s just about everything associated with human body temperatures to ponder until the cows come home
https://en.m.wikipedia.org/wiki/Human_body_temperature and in a broader sense
https://en.m.wikipedia.org/wiki/Thermoregulation.
Save to say we can’t simply account for those who are way off the normal scale and need huge warm sleep systems than normal. I’m just very glad that I have always been a warm sleeper surviving with 0 C rated bags in -10 C conditions with a standard liner and undies, saves heaps if Base weight and life is simpler.
Personally I think a lot has to do with one’s own metabolism and internal body volume to external skin surface ratio heat loss rates and possibly some people with dicky hypothalamus mechanisms that need medical attention
The longest down sleeping bag I had was a 1985, 1.5kg, 50/50 feather and down Fairydown NZ bag rated at -9 C, which was still good just last year and perhaps lost about 10-20% of its loft and warmth by 2018. Quite amazing testament these products can last 34 years plus. Used this bag over the last 35 years of on and off travel, backpacking, local car camping, several 23-30 day Nepal treks to EBC and Annapurna. Cost $209 in 1985 and sold in Gumtree last year for $120, not bad really.
Now have ultra light weight - 2 C and 0 C Western Mountaineering bags with poly or silk liners and recently Mike’s on this forum custom - 2 C down light weight quilt, all around 520-600g products.