Maria Zanini has found and shared the amazing truth here: The Awesome Secret About Captcha. It isn't as random as it looks: every time you give those squiggly words your best guess you're part of an international project to digitise books.
And now the fun parts: firstly there are some amusing examples of where two random words get together at the wrong place at the wrong time: a reCaptcha on a US church site came up with: 'Bad Christians'
CaptchArt where artists come up with illustrations for these random couplings, some of which I've shown here.
All of this makes me feel a whole lot better about captcha - how about you? If you want to get rid of it go to Smitten Image who will tell you how. Also, does it ask you to do it on this site? It shouldn't!