Ben Edelman produced a detailed report released through the Berkman Center for Internet & Society that shows how Google's SafeSearch feature not only blocks adult material but tens of thousands of sites not pertaining to adult content.
Based on Google's description of SafeSearch they filter for keywords and phrases, URLs and Open Directory categories. Just like traditional spam filters, unless it's heuristically based, expects lots of errors. I'm sure Google would rather error on the side of extreme caution rather than have occasional incidents of missed adult content spring up on little Billy's computer screen.
Heavy statiscial analysis on each page is probably too costly for Google at the moment. A two stage filter might prove more effective. Of those classified adult material, run them through a more intensive analysis to be sure and leave a good margin of error to keep out the false negatives.