Here's something interesting. If I go to the ReCaptcha demo page in Chrome that is logged in to Google, I get all house numbers, a lot of which seem like easy OCR. If go to the same demo page in Incognito mode, I get the two word version instead, like this blog is complaining about.
It was one thing when they were digitizing books. I refuse to help them improve Google Maps. I have never correctly entered in a map number. As long as you're only off by one digit they are accepting
And if you start entering some number that are close to the correct answer most times it's accepted. But if you do it like ten times you might be switched to the two words version.
Leaving no captcha would be a hint for spammers to provide requests with valid cookies thus simplifying the task. I assume that's the reason for increased difficulty in incognito.
It is based on the google tracking cookies, not being logged in. I don't have a google account, but I still see the house #s on firefox, until I flush their cookies.
http://www.google.com/recaptcha/demo/