The key difference is that we're not asking "who won this competition", but "who won the World Record" - what are the conditions of the best ever result. Since no-one has done better in 20 years, that really suggests to me that it has to be a '20' on that roll.
I'll yield the point, as any difference would probably be less than a centimeter, and this is an issue of inches.
Joe has a hard limit to how far forward his foot can be. Indeed, given the requirements of leverage, he's actually better not having his foot "over the line".
If Joe is leaping from the ball of his foot, his toes can be over the line. That gives him inches, depending on how long his toes are.
This is true, but I'll bet that World Record jumper didn't land standing on his feet. Per RAW, Joe does.
If Mike Powell fell backwards, his distance was measured to where his backside or hand landed, depending on which was further back. It seems unlikely this is the case(as it would be very rough on his score), and if it is, then his feet are well ahead.
As for falling forward, upon doing so Joe can roll up to his feet. Mike Powell wouldn't have any reason to do this, but Joe would.
This, however, is true, and is a point I will concede.
That is very gracious.
However, even given this, Joe is still superhuman. That World Record jumper was an extreme specialist in that one event. Joe can swim, climb, run (and high-jump) just as well as he can long jump. He doesn't even need to rest between these many feats.
I suspect that the World Record holder has much greater consistency. His jumps are probably reliably within the 25-30 foot range, while a jump by Joe could land him anywhere from 10-30 feet from his launch point.
As for resting, Joe is likely to want a rest after he fails any of his attempts(and takes the resulting damage). Don't forget, he is still failing them some 80% of the time.
Now, the game doesn't support these extreme specialisations, and rightly so. The more narrow the categories become, the more things get excluded (since there isn't infinite page count). But it does give rise to the problem that 1st level characters aren't just "a cut above". They really are superhuman.
They are less reliable than Olympic athletes, but capable of a wider range of feats. That's just different, not straight better.
And Joe isn't even a terribly unusual 4e character. The game assumes that an '18' in the primary stat is the norm, with a '20' being common.
Actually, Joe
is terribly unusual. A 20 is expensive, especially for a human, who only gets to boost one ability score. And while training in Athletics is likely for a fighter, it's not certain.
But most significantly, who takes Skill Focus(Athletics) as a level one feat? This idea relies heavily on that selection, and in the PHB1 alone there are some 9 other feats that are at least as appealing for which Joe
must qualify, in addition to any others his ability scores may qualify him for. That choice alone makes Joe very unusual, and with the other two factors being reasonable but not certain, there's no way this combination is anything but terribly unusual.
So we get a game where every Fighter is The World's Strongest Man, every Wizard is an Einstein, every Bard is an Elvis, and every Warlord is a Julius Caesar. And all this at 1st level, before they've done anything! (Still, I suppose that does negate those "1st level Warlord" arguments we had back in the day.)
As bganon points out, this is typical for the great heroes of the age. The PCs are among the best of the best by design.
So, yeah, I'm going to stick with my assessment that this sort of performance by 1st level characters is an absurdity in the rules.
You do what you must, as do we all.