Your data conflicts with
usatf.org -- I'm gonna go with the official track and field site.
I disagree with Str 16. These are people still in puberty, probably not fully grown. I doubt many of them can lift 480 pounds. I also disagree with saying they are proficient. They are on their way to proficiency, but at that age, they are probably not quite there yet.
But the point is, there are people that age and only slightly older getting long jumps in the high teens and low 20s. And these are records. There is no mention of averages, or the range of results recorded at track meets. I'm sure the difference between an average jump and a record is a matter two feet or less, which means those kids are routinely jumping well over 10 feet.
I'm pretty sure I could manage a 10-foot running jump with no problem, and I'm neither strong nor athletic.
I'm certain you're dead wrong. The average running long jump I've seen in my 6th graders was about 5'. The ones on the track team were pushing 10'. (I was there for the Physical Fitness tests). Those records are set by the top 0.01%. They've been putting in (typically) 2 years developing the skills. And I'd love to see video of you attempting a 10' running jump with 20 pounds of armor on... let alone the 30-40 pounds of stuff the PC's I've seen are carrying. And I'm fairly certain some of them are easily Str 16.
I'll point you to some actual competition records...
http://www.asdk12.org/schools/romig/pages/track/be4-178b.html
http://www.asdk12.org/schools/romig/pages/track/be4-178g.html
These are from 2009, but they show the distances amongst the best capable from 2 middle schools.
the 5th place girl in that match didn't even break 8'.
http://www.athletic.net/TrackAndField/Division/Event.aspx?DivID=49859&Gender=M&Event=17
http://www.athletic.net/TrackAndField/Division/Event.aspx?DivID=49859&Gender=F&Event=17
Statewide Alaska Track and Field stats from 2014 show high school results mostly between 10' and 15', but some as low as 1'3"... (obviously, he missed the end of the run-up. This would be a failed Strength (athletics) roll...)
Even so, that's still the best from each school. If you ignore the obvious failed-to-make-the-roll cases, you can see a best performance from each student...
Given that the local high schools in Anchorage are about 14,000 students, and only about 1/4 the total competitors, so, maybe 50ish (I'm too lazy to count the totals) are competing in the long jump. My alma mater runs 2100 students, about 55% male, so about 945 girls, and 7 competed in long jump. Less than 1%. Not even half their team, either. So, the girls probably representing most of the top 1% (as some who would excel fail to meet academic requirements). Lots of 8's and 9's at the high school level for women, and those are the most competitive in the sport. The top 1%.
Let's assume, for sake of argument, that they're getting advantage for prep and relatively perfect conditions.
That means roughly 39/400 results are 20's. If we assume only level 1, and rolled attributes (age 15 is a viable starting PC age, per the rules), it's possible to have a +6 on the roll, so we look for peak performance. Hell, the track team alone is mostly going to be mostly 16's to 18's...
That several aren't even making it to 10'... misplaced footing, or a bad day, or whatnot, but it shows there's a range of performance. (Also - the school competitions are best of three tries for long jump.)