This behaviour is caused by reCAPTCHA.
reCAPTCHA uses scanned images from books to generate CAPTCHAs (more info here). The way it decides if a response is correct is to check it against other previous responses to the same CAPTCHA (because the system doesn't know what the true correct answer is). To account for this, reCAPTCHA has some margin for error built into the check - so if you get one or two letters wrong but the rest right, you will probably pass.
You can try this for yourself on the official demo page (same link as above) - get a couple of letters wrong and it will still say you are correct.
If you want to have only one possible correct solution to CAPTCHAs, you are better off using the default text-based option.