Notice that although we used only the 20 applicable paraphrasing rules with the highest scores to generate the S, C pairs, less than half of the candidate paraphrases (C) were considered good, and approximately only 20% perfect.
applying paraphrasing rules (even only those with the 20 best scores) to each input sentence S and randomly picking one of the resulting candidate paraphrases C, without any further filtering (or ranking) of the candidates, would on average produce unacceptable paraphrases more frequently than acceptable ones.