I may be mistaken, but I've found reCAPTCHA to be often forgiving, sometimes the words are just not clear, and I just type what I think it is. I don't remember ever being incorrect on the reCAPTCHA.
reCAPTCHA only knows what one of the words should be, so you only have to get one of the two words right. The other word is being transcribed by crowdsourcing.