A 10 represents an average person of average intelligence. That's what it represents in D&D. That's what it has always represented in D&D. It has nothing to do with high school exams.
Here are the other CR 13 creatures.
Nalfashnee: DC 15 Success rate with +7, 60%
Adult White and Brass Dragons: Frightful Presence DC 14, Breath and Wing DC 19 Success rates: 65% and 40%
Storm Giant: DCs 17 Success rate: 50%
Vampire: Charm DC 17, Grapple DC 18 Success rates, 50% and 45%
+7 is average. Some things are easier. Some harder. Some right on the money. If you go to 20 it makes it all easier, but the game isn't designed with 20 in mind.