I don't get it. They watch how I interact with the website to figure out if I'm human?

To counter this, last year we developed an Advanced Risk Analysis backend for reCAPTCHA that actively considers a user’s entire engagement with the CAPTCHA—before, during, and after—to determine whether that user is a human. This enables us to rely less on typing distorted text and, in turn, offer a better experience for users.  We talked about this in our Valentine’s Day post earlier this year.