If you take any population, and remove those too weak to survive to adulthood, the resulting population will indeed be "stronger" than the assumed population of those born. . .but that is the average. . . (That's essentially the definition of one of the core principles of evolution).
The average will only fall on the border of the 3rd and 4th quartiles when a population is under extreme stress, and therefore the species is in transition to a new average. . .technically, the average stats within any population should be 40-60 due to normal deviation from average 50, with all other variation being rolled into stat bonuses. If at any time the average stats of a group were to actually reach 75, then they would be a new population or sub species and their racial stat bonuses would rise until the average stats dropped back to 50 (40-60). . .the same would happen if anything caused average stats to drop to 25, just with the racial mods going down until the average went back to. . . average. (That's one of the other central concepts of evolution, in game stats)
Reading the books, among sentient species, the logic, apparently, is that:
A) Shopkeepers, farmers and the like have stats from 40-60 or 400 points (50 average). . .they are, indeed, average.
B) NPCs should be generated almost as if PCs with 550 points (68.75 average). . .assumed, like PCs to be above average exceptional beings, the cream of their population.
C) Purely combat oriented encounters with statistic blocks use 600 points (75 average). . .these are fairly elite representatives of their species.
Most animal/monster encounters are type C. . .personally, as a GM the biggest justification for that would likely come from the fact that they're mobs run by the GM, that they have no skill variation to speak of, merely a couple of tactical numbers, and they're jacked up a bit to weight things a bit in their favor in exchange for not going into a level of detail to create a generic character sheet for them with a realistic spread of skills, akin to type Bs. . .They do appear to be over-weighted and more powerful than the average bear (or goblin) should be. Of course, as is, all those type Cs can do is move and fight.
If you ever wanted to actually go to the work of it, you could easily re-create all the monsters as Bs using the racial stat mods and one of the spread sheet. . .but that would still give you fairly aggressive, "cream of the crop" representatives of that race. . .Goblin warleaders, The Alpha Wolf of the pack, etc.
You could, presumptively also do the same thing to create "average" members of that species using the 40-60 stat generation method for non adventuring people, which would give you the non adventuring stats for that race. . .the Goblin shopkeeper, the pet dog, the average horse in a stable, that kind of thing. . .
Does seem like a lot of work though, it's tempting to be lazy and use the stats in the stat block.