Check this website: https://developers.google.com/recaptcha/docs/troubleshooting
reCAPTCHA is Accepting Incorrect Words One of the words displayed by reCAPTCHA is from an old book which is being digitized. As such it is not graded -- it cannot. Indeed, occasionally it may be blank, or just a bit of noise.
On the word that is checked, the user is (by default) allowed to be off by one letter. It has been found that this increases the user experience while not degrading security by a large amount. This is tuned dynamically based on many factors.