We should remember that this is only 26% of test takers, not all American high school students. Indeed, I would say that the former group is not reflective of the latter.
I have heard of high schools administering the ASVAB to all of their students, but nothing like that ever happened where I am from. Indeed, taking such a test is probably looked down upon by most around here, especially when a lot of parents are more concerned about their kids studying for AP tests (i.e. kids are on the track to go to college, not the army). I would venture to say that the top ~10% of students never take the ASVAB.
Secondly, I would expect many of those who are seriously considering joining the Army out of high school probably did not do well enough in school to go to college, which means ASVAB takers are not as smart as the average student.