You aren't supposed to realize it or do it. Google uses the human's work on solving the unknown word to either get street numbers from photos or digitize books.

However, the whole point of them having the humans do that work is /they do not know the correct answer/. Since they do not know the correct answer they cannot be basing the test of the CAPTCHA on it. From there it is not a big leap to surmise that you are only actually being tested on the word that actually looks distorted on purpose.