The human candle explanation
While spontaneous human combustion might be a compelling answer to these mysteries, there’s actually a widely accepted theory that explains away almost every irregularity. It’s a phenomenon called the “the wick effect,” and it’s both a compelling explanation for most cases of suspected SHC and very, very gross.
According to the theory of the wick effect, the human body acts sort of like a candle, with the person’s clothes (or some other draping, like a blanket) serving as a wick. As the clothing burns — ignited by, say, a stray cinder from a nearby fireplace, or the ash from a cigarette — it splits the skin and melts the victim’s body fat. The fat then soaks into the fabric and becomes another, constantly renewing source of fuel that keeps the fire going and creates extra heat. If you’ve ever had a grease fire in your kitchen, it’s basically like that, except with a human body at the center instead of deep-fried stuffed peppers.
In experiments done with pig flesh (which is close to a human’s), fires that start this way tend to burn straight up, singing the ceiling, and also progress fairly slowly, leaving the most of the surroundings intact. This also explains the greasy residue that the fires leave behind — it’s liquefied body fat. That also explains why the largely fat-free hands and feet tend to survive these fires, why SHC tends to affect the overweight, and why the source of the fire is often unidentified. At such extreme temperatures, whatever started the blaze is incinerated right alongside the body.