The earliest ones that asked you to type two words were also used to help digitize the Google Books library, for cases when the OCR wasn't quite able to match a word confidently. That's why you'd get a pair, one that was clearer but "jumbled" to be unreadable to computers, the other looked more like a fuzzy scan (or occasionally just a complete mess of misplaced ink), though both had a wavy effect applied to them. Only the clearer one was needed to pass the test, the other was just free training for their OCR process.