Saying "only 1 word has to be entered correctly'' is pretty outdated. I'd say you need to have a +90% accuracy on the captcha for google to give it a greenlight.
There are also many cases in which the EXACT match is needed. However, this is not written on stone, as I remember a few months ago google was trying a new type of recaptcha with 2 set of numbers, and back then they did allowed to type only one set of numbers correctly. It's a matter of trail and error, but bottom line, if it's for sensitive stuff I'd just recommend a human based Captcha solving service.