It’s important to note the way reCAPTCHA works. Each user (or bot) is presented with a control word, and a word unrecognized by OCR. This control word is already known to Google (who runs reCAPTCHA). If you get this first word right, it is assumed that you get the second word correct as well. So, in reality, you only need to guess the key word correctly.
The author explicitly addresses your point and if you looked at his examples, most are very difficult for both words. In many of his examples, the control word is distorted beyond reasonable recognition, and the new word is cut in half or worse.