It's actually pretty normal. Kids can "fail" within a specific range of normal. My eldest has spectacular vision but she "fails" the eye exams at the pediatricians office because frankly she just doesn't concentrate very well and fidgets. She likely will need to wear glasses when she's older because of genetics, but for now her eyesight is excellent based on what we know she can see. There's also the fact that she was a very, very early reader and she hasn't stopped reading since she started and there's a causal link between early reading and wearing glasses at an earlier age.
I was an early reader and had to start wearing bifocals by junior year of high school but probably should have started wearing them sooner. I was getting increased migraine frequency for nearly two years before I got my glasses and getting them did decrease my migraines. Since my daughter does get migraines we do keep an eye on the frequency and if she's squinting and other warning signs for needing glasses. Actually, with her recent issues with homework and school work, I may have her eyes double checked even though she just had her pediatric exam in her annual physical.