Well, I do not beleive that you failed for just one letter. Maybe the looked at the whole picture.
Maybe your young and you already have strong prescription already and if you are a borderline case maybe they are a bit stringent then.
My question is, do you feel that the schnellen where more difficult then the other ones at you optrician. The ilumination pays a BIG role.
I really don't think that you failed for just one letter. I beleive that many of us have said the wrong letter sometimes.
Also to considered is to wear contact lenses. They give you a bit better vision then spectacles. If I where you I would call CAA and ask for an explanation. Also I would go every week to a optrician and then send in the evidence again.