That's very true. I've noticed that optometrists generally don't actually ask me to read out the letters to prove what I can see.

They usually just use it while making incremental changes to the lenses and asking "which is better, 1 or 2". There would be no benefit to lying since I want my prescription to be accurate.

I guess it's different when the chart is actually used for a "test", like when someone is trying to become a pilot or whatever. There must be a little guesswork to get the best possible "score"! Then again I know there are other ways to measure the refraction of the eye, so I'm sure they don't rely on the chart alone.