The World Wide Web Consortium (W3C) in the article, ‘Evaluating Web Sites for Accessibility’, states,
“No single evaluation tool yet provides comprehensive information or captures all problems with regard to the accessibility of a site; therefore evaluation involves a combination of approaches.” http://www.w3.org/WAI/eval/
Of course this is not new, as far back as 2004 The Disability Rights Commission highlighted issues related to testing website using the W3C Web Content Accessibility Guidelines,
“It is very significant that the majority of those Checkpoints that this investigation found to be the most important are qualitative, in the sense that they require the exercise of human judgement. Automatic testing tools alone cannot, therefore, verify effective compliance.” http://www.drc-gb.org/publicationsandreports/2.pdf
Evaluating website accessibility is an art not a science – it can’t be reduced to running your site through Bobby and keeping your fingers crossed that everything will be ok.
Even today automated tools still can’t be relied upon to check the accessibility of websites; you will always need a human being with some knowledge of the subject to do that.