Details of their experiment and other breakdowns of data can be read in their report here. The author states:
Our tests yielded some interesting answers: First, censorship levels across 15 different BSPs varied even more than expected. Second, a great deal of politically sensitive material survives in the Chinese blogosphere, and chances for survival can likely be improved with knowledge and strategy. Third, censorship methods vary greatly from company to company, implying that companies do have at least some ability to make strategic choices. These choices are not only about how to balance relationships with government and users, but also about the extent to which BSPs value user rights and interests.