Earlier quoted context omitted.
ReCaptcha only requires one (1) out of two (2) words to be correct in the challenge. It presents one known-by-the-system-word, and one not-known word. If you get the known word correct (the easier of the two to read) then it passes the challenge. ReCaptcha then pools the answers for the second not-known word and after pooling thousands (or more) responses, then that word becomes "known" based on the average answers (…
Again, sorry, I've got to point you back to the original article. The author explains the details of reCAPTCHA's known/unknown word-pair clearly, as you have done, but goes on to explain that the impossible-to-read word was actually reCAPTCHA's "known word", so the CAPTCHA was impossible to pass.
I do have to admit most of those are cases where both words are difficult or impossible. But we can at least assume that the easier of the two (the one not cut in half) is the control in most of those.