I'm a bit of a hypocrite demanding clarity from the NIH but not being clear enough myself. :P I'm in full and complete agreement with you that vague terms should be avoided. "Mean lifespan" would be better than both life expectancy and longevity. "Top 10% of lifespan distribution" would be clearer than longevity, and probably more informative than "maximum lifespan".

Maybe I should have said that the number of people living to be older than 130 isn't going up, instead of 100. Yes, there are more centenarians now. Yes, there are more people over 80 now. None of that means that we've increased the maximum possible human lifespan in any way. All it proves is that we've decreased the number of people who died before they could have, right? Better medicine, fewer murders, safer cars, cleaner air, less food poisoning. All these advancements help us "live longer", and yet none of them increase the maximum lifespan. We are getting asymptotically closer to the maximum possible, more and more people are approaching the limit, but there is no evidence yet that the limit is moving or has ever moved, and that's all I want to be clear about.

I'm mostly making an argument to counter people (not you) who are, for whatever agenda, intentionally suggesting that increases in life expectancy are due to increasing maximum lifespan. It's a common tactic, and it's a falsehood. The problem here is that the NIH is doing it a little bit. They did paint a picture of huge variability in life expectancy and then conclude that diet and exercise are the major solution.

"But nearly three-fourths of the variation in longevity is accountable to behavioral and metabolic risk factors, including obesity..."

I would be willing to bet that this statistic is citing longevity as I've described it, and not life expectancy. I don't believe it's true that 3/4 of the variability of life expectancy is due to diet and exercise in combination with genetics. But "behavioral" risk factors is super duper fudgy, so I have no idea. Is dying of road rage or a skydiving accident the kind of behavioral risk factor they're talking about? I don't know, because they quoted a statistic that you could interpret to mean almost anything.

If that's the case as I suspect, then this article has knowingly and intentionally mis-used the terms and left a misleading impression without saying something technically untrue, precisely because the popular lay-person's understanding is that they're synonymous.

Scientific references for longevity being constant... I don't have a definitive source, I've mostly had many long discussions about this with my brother who just finished his PhD in anthropology and told me about longevity being constant. Before that I was under the mistaken impression that quotes you get in school about historic people dying at age 35 meant that nobody lived past 40. Lots and lots of people believe this, and it's not true. Here are a few things I got poking around just now:

http://www.livescience.com/10569-human-lifespans-constant-2-...

http://www.ancient-origins.net/news-evolution-human-origins/...

The table in Wikipedia's article gives some indication too (Life expectancy at older age):

https://en.wikipedia.org/wiki/Life_expectancy#Variation_over...

The stats about huge increases in life expectancy for people over age 10, 15 or 21 are all trending in the direction of people who make it to 20 can expect to live to near 80, plus or minus. Much of the "proof" comes from historical writing, and not from scientific evidence, according to my anthropologist brother. But it doesn't take an average to prove people were often living to be four score years old, it just takes a few samples & stories to know it was somewhat common, right?