First of all, if we are to assume that an 18 is the highest possible human strength, forgetting entirely about percentiles, who has such a strength? I'm going with this attribute to start because why the hell not...simple math demonstrates that there is a 1/216 chance of getting an 18 on three dice. Fine. No one ever rolls three dice for stats anymore, but whatever, I gotta start this post somewhere. That's a little less than 1/2% chance of having the maximum strength possible. So, 1 out of 216 people will have an 18, right? Does that make any sense whatsoever? Not really. Check out this webpage of weightlifting performance standards. I fall somewhere between Advanced and Elite in all the lifts, which means I obviously do not have an 18 Strength. Further, I know people who exceed the Elite value by quite a bit. Sure, they might be on steroids or whatever, but they're still stronger than me, and stronger than average people by at least a factor of two. If an 18 is supposed to be the best of the best, we're talking a lot fewer than 1-in-216 people. Probably closer to 1-in-a-million. If we take the categories and divide them up into stat ranges, we might get something like:
There's a lot of lumping together toward the higher end of the scale for good reason: to most people, there's no discernible difference between someone 1.8X as strong as you and someone 2.2X as strong as you. If you're the average desk jockey benching 150 pounds max, someone who can bench 300 will look like a monster. The 400 pound bencher might not look any different, and really, there isn't a whole lot to separate them in reality, even though there's a 25% difference between their scores.
Want more granularity? I figure if we do a 6-36 scale (6D6), the highest score, 36, has a 1-in-1679616 chance of coming up. Definitely much more reasonable given what I know about strength competitions. The super elite lifters, Olympic-types for instance, are very rare. Even most professional athletes will never have this level of strength, and quite honestly don't need it. Olympic lifts are extremely technical and as much about technique as they are overall strength; thus the score of 36 not only indicates actual musculature but the ability to use that musculature to its utmost capability. Deadlifts are not an Olympic lift, but I'll use it as an example. I can max Deadlift about 565 right now. This is fairly good, nearly Elite by the chart. However, if I'm not completely focused during the lift, I manage perhaps 505 or even less. I've had bad days where I physically felt great but mentally just was not into it, had trouble with 485 or even less. There have been times where my mental state was completely focused, although my body was fatigued; set a Personal Record. Does this mean I had a 16 Strength on the 3-18 scale one day and an 18 the next? Not even close. It probably does mean something more like my Strength is 16 or so and I'm capable of outperforming someone with a 17 sometimes who has an off-day, but the guy with the 18 will generally ALWAYS beat me because his mental focus is better than mine. It also means that the scores themselves are ranges, hence the whole "greater granularity" crap at the beginning of this paragraph. I'm not going to roll 6 dice for a 6-36 range stat, that's just stupid. All that does it make it less likely to get the 18 because people in Real Life don't have 18s. Who cares? It's a game, right? It does mean, though, that Halflings with 18 strength are reasonable in-game. Sure, the guy is 3 feet tall, but he uses his muscles at 100% capacity. In Real Life, there are some very small people who are insanely strong so I don't have a fundamental problem with fantasy races being exceptionally adept at using their muscles. People don't say shit about elves being smarter than humans, do they? Are their brains bigger? Not really...
Using the pseudo-logic outlined above, basically either an 18 is extremely rare and PCs are just so awesome they get a really really good bonus to being exceptionally badass OR everyone in the D&D world is a lot closer in capability than our own reality. Who has an 18 Intelligence in Real Life, for instance? There are probably a few people you could think of, maybe Newton or Leonardo. In D&D, most magic-users have close to an 18, which means not only are they smarter than the vast majority of people, they are as smart as the greatest minds in history. I'm fine with that, I really am. BUT: what about some farmer the DM rolled up with an 18 Intelligence. You could say he's not "living up to his potential", but given what I said earlier about an 18 not only having great ability but also utilizing it properly, that farmer really doesn't have an 18. It also means, perhaps, that a 16 Int wizard might be able to train himself to an 18. Maybe?
I don't think the 3-18 scale can be rectified for what it is supposed to model if we roll everyone's stats on 3D6. Now, supposing we roll 2D6 for the average rube. That works a little better, but it also eliminates the burly farm hand and the wise old woman from the mix. Given that, it seems like rolling stats is entirely the province of Player Characters, and the DM should assign stats to everyone else as necessary. Even then, does it really matter? Who cares if the shopkeeper has a Con of 9 or 12, he still only has 3 hitpoints. No need to roll.
The real reason I even brought this up is because I like reading about how people model their Real Life stats. Everyone who plays D&D has an 18 Int even though I've never heard of them. 18 Cha? Well, why aren't you a movie star? All those 18 Str guys are doing strongman competitions and competing in the Olympics, they're certainly not typing up stupid blog posts while watching Monday Night Football...