Yaarel
🇮🇱 🇺🇦 He-Mage
As @EzekielRaiden is already pointing out,just because there's mass variation in a species it doesn't also mean that there isn't also trends and strengths that appear in that species, especially when in comparason between one species and another, so sure you can have the dwarven species that on average varies between 8-17 CON but next to elves or halflings who's species averages around 6-15 CON that's still a relative +2 higher average, even if you are that dwarf with 8 CON.
Even if, on average, a population of Dwarves has a higher Constitution than a population of Elves, it is still untrue to say that every individual Dwarf has a Constitution score +2. Rather, the Dwarf population has a higher frequency of individuals with a high Constitution. Therefore these individuals increase the average of the rest of the population. But there are still some individual Dwarves who have a Constitution score of 6. And there are still some individual Elves who have a Constitution score of 20.
This goes back to the background. What are these individuals doing that makes them relatively tougher than others of the population? Perhaps these Dwarves are miners, smiths, or mason construction workers − jobs that require extensive endurance.