whiles she is not an optometrist, she did say "because each is different, try to find one that look best". And on the page before, you can see that each have magnification strength level.
Although I do think that the author could have add one more page to make the prime minister and the king try a different one that end up better. I think she didn't think it through because it the first time the king try on a glasses, everything look better instantly on the first try.
If you wear glasses and still remember the first time you getting the test You know what I'm talking about.
Me: "Holy S, you suppose to be able to read that?! Is this how normal people see the world"
OP:*click* " How about this"
Me: "Holy hell, it can get sharper?!"