An elf isn't just "good at dexterity." That's not how math works. They get a plus two. Which tells us that the average dexterity of all elves is higher than the average dexterity of all, let's say, githyanki.
But it doesn't tell us anything about individual elves versus individual githyanki.
An elf can roll a 3 and get a plus 2, equaling 5 in dexterity. It happens. That elf has less dexterity than the average human.
3d6 creates a bell curve. 3d6+2 creates a different bell curve. They overlap. Some elves have less dexterity than other races even if the average tends to be higher. And some githyanki have a much higher dexterity than the average elf.
