I wonder if it has anything to do with the fact that you eat less food when it's particularly spicy? Fasting/caloric intake reduction and better health/longer life have been correlated for a while.
Source: I eat quite a lot of habaneros and thai chilis, mostly because those are the peppers that taste strongly "spicy" to me on a day to day basis. If I eat too much on an ongoing basis, even the habaneros start to lose their spiciness. Most people have a "spiciness" threshold so low that the pepper is barely detectable to me. I was not born this way, peppers have been a regular part of my diet because I like the flavor and that confers a natural immunity to the effects.
That's unlikely. People will adjust the spiciness before adjusting their usual portions. Don't forget that habitual users will develop tolerance to capsaicin by reducing the number of receptors, so what may seem as a high level to someone may be perceived as normal by someone else.