what are the most consistent stats that come up?
Who cares? Seriously. Who cares what is "most common"? Are we only and exclusively playing the incredibly rare "perfectly average elf"? I certainly don't. I play my character, who will often be significantly, even radically different from other elves (well, usually I play dragonborn, but you get my meaning.) I play characters that are outliers in at least
some ways, because toeing the line of "perfectly average" is boring.
Further, keep in mind: if you measure more than about 5 or 6 things on a given human being, even with an
incredibly generous definition of "average," you are effectively guaranteed to not find anyone who is average. This was discovered by the US Air Force in the 50s when they were trying to make airplanes and skin-tight garments that would fit the "average" airman. Turns out, even if you only choose
ten measurements, then out of over 4000 different measured airmen, there were exactly zero that were average in all ten areas. You can see this quite simply: Let's say "average" people are in the middle 50%, yeah? That's quite a generous definition of "average," I should think. But that means any given person's likelihood of being average on any given metric is...well, 50%, by design. 0.5^10 = 1/1024. So even if you consider the middle half to be average, you'd only expect about
four "average" airmen in 4000!
would it not be a equivalency?
A sickly elf has a start of four a very sickly dwarf has a stat of four.
all will eventually produce more or less andy stat combo that will result in an individual living to a age where they are recorded but what a culture thinks as baseline might be really different.
Not at all.
On the one hand, the range of variation within a given species is
vast. The shortest verified human being was 21.5 inches tall (1'10.5") at the time of his death in 2015, and he was 75 at the time, so it's not like his height reflected any severe medical complications (his cause of death was not reported, but he had apparently contracted pneumonia sometime before his death, which may have been the cause.) The tallest non-pathological human (that is, not someone suffering from gigantism) was 7'9". Outliers, even perfectly healthy ones, can be vastly far away from the average.
Further, there are certain hard limits one cannot pass without...well, death. Who cares if it is a "very sickly" dwarf vs a merely "sickly" elf? They end up having the same stats. But
that's not possible when ALL dwarves have an innate +2. The sickliest elves are sicker than the sickliest dwarves, even though that isn't how real populations actually work--there's a hard cutoff at
some point. Likewise, 5e explicitly has a hard cutoff at the top end: no mortal can be more than 20.
These hard limits ensure that, because populations are weird and variable and have outliers, you'll definitely have some sickly dwarves who are exactly as sickly as an elf-equivalent.
The best way to represent this, then, is to not make Constitution
mandatory for dwarves, but to still have it be a common inherent feature. And that's exactly what the "+2 Con
or +2 Wis" option does. It ensures that dwarves will still be known for their hardiness (after all, implicitly, about half of them will be hardier than the theoretical average being), but
doesn't force the unrealistic and unnatural requirement that EVERY dwarf is simply a cut above EVERY elf of similar life and experiences.