Seeking for advices on results from automated web evaluation tools

Dear All,

I'm writing a research paper to investigate the inter-reliability of
automated evaluation tools. I used two automated web evaluation tools to
scan the same web pages. The findings indicates there are highly
discrepancies in the results between both tools although they're based on
the same standard (WCAG 2.0).

I'm new to the field. Any explanation for such a case?

Thanks!