On one of our school sites, ces.buncombeschools.org, the school logo shows up under "all images should specify an 'alt attribute'" more than 500 times, even though it does have alt text. These false positives are keeping me from being able to see actual problems.
I reviewed your site and the result of the alt text errors. The page pulled in by our application has some different content than what is on the site now. That can be either a recent change or there is code manipulating the result. Since the error is on the logo, it appears to be on every page, thus the high page count.
The page source read by the application is missing the alt tag. So it isn't really a false positive, it is the variance between what DQM read at time of scan and what the page source is currently. Recommendation would be that we do a full rescan of the site, which includes an initial purge of the current content stored in DQM to ensure the content is fresh as the page exists. Please open a ticket if you would like us to start that for you.