Kid passed nurse eye test but really has bad vision?

Ok, so I need to know if this has happened to? Don't need to know just want to know that I am not the only one... so My oldest boy is in the 6th grade,and HAS NEVER failed one of those eye test at the nurses office. However, we took him to the dr. bc he scratched his eye, so they do the test.. and yep.. just what you are thinking he failed... BIG TIME.. not just a lil but we are talking majorly noticeable.. so what's up with that? I have even asked the nursed he would ya mind checking the Oldest boy, he mentioned he is having trouble seeing.. just curious if he fails the test.. and each time I got he passed with flying colors, so did she never do the test or did he really pass? I feel like such bad parent now knowing how bad his did on the eye test... so have you ever heard your child passed the test and now he totally has eye problems, probably did the entire time?? What are you thoughts? Have you or someone you know had perfect vision then just out of the blue say a few months and your vision totally changed?