I am not an optical expert but I take ds to the eye clinic at the local hospital every three months to see an orthoptist as he has a syndrome affecting his eyes.
When we go to the hospital, we see the orthoptist, not the optician. The orthoptist looks for coordination of the eyes, not long/short sightedness and does a different set of tests to the optician.
A couple of times, ds has failed the glasses test part of his examination but this is not seen as significant unless it happens a few times in a row due to his age (he's 4.5). I think this is due to the changes due to growth.
HTH