well i have a friend who has done optometry at university, and is now doing her pre-reg year.
she told me that opticians, on purpose, make the glasses ever-so-slightly stronger (than they should be), in order to "guarantee" that you do come back next year (so that their business does well too) because if your glasses are stronger than they should be, your eyes will adjust to that strength, and will therefore get weaker and weaker (meaning you will need stronger and stronger glasses each time)
thats not against the law or anything btw.
meh, i'm just gonna have laser eye treatment (Soon hopefully)