I had no idea the two were different, so thanks for making that distinction. For anyone else's sake, I'll share what I learned:
Life expectancy from birth weights times of poor medicine and prenatal care towards the low end of the life expectancy scale because of much higher infant mortality rates, whereas in modern times life expectancy from birth would be weighted the opposite.
So basically, if you filter out infant mortality and other "first years of life" deaths, you get a more accurate picture of how long someone could actually expect to live if they make it past the initial thresholds to human existence.
Yeah, the problem is basically that historically you're most likely to die either very young or fairly old, but averaging out the ages at which people die creates the opposite impression — that you're likely to die sometime in your late 30s to early 40s, which was never actually the case.
Life expectancy from birth weights times of poor medicine and prenatal care towards the low end of the life expectancy scale because of much higher infant mortality rates, whereas in modern times life expectancy from birth would be weighted the opposite.
So basically, if you filter out infant mortality and other "first years of life" deaths, you get a more accurate picture of how long someone could actually expect to live if they make it past the initial thresholds to human existence.
At least that's what I've grasped thus far.