reCAPTCHA’s verification uses several factors to determine the chances that a user is a human, not just the answer provided. We allow true humans to make mistakes in solving the challenge, while punishing bad bots even if they submit a correct answer.
It is expected that, if the system determines you’re likely a human, it accepts your answer despite knowing that it’s an invalid one. In fact, this feature is necessary to be able to combat spam effectively - if we always require a correct answer, it would be easier to create an automated solution to bypass reCAPTCHA challenges. By accepting invalid answers (and sometimes rejecting valid ones!), creating such a bypass gets much more complicated for spammers.
Google ReCAPTCHAs Will Occasionally Accept Incorrect Answers Because Real Humans Make Mistakes. Fun bit of trivia regarding those “click on all the squares with street signs in them” prompts you tend to see on a login or sign up page: