The second word is actually a word scanned from a book that was not recognized by the OCR software. If you get the actual captcha word right they assume the word scan is right and update the online book scan word. That is the "Re" in Re-Captcha, using it for more than just security validation.
I can tell you that without captcha the site would get spammed into oblivion as I have had to resort to it on many of my websites as well just to slow down the stem of spambots etc.
Hopefully there will be a better way someday.--
Onsite Computer Support in Memphis