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 (and then that word is "digitized" and used by google maps, or ebooks, etc).