If you click the View errors link next to the page being reported on, it opens the same page in the official W3C validator. These results should be the same. If they aren’t, you will need to contact us and let us know.
Why does this happen?
Sitebeam puts so many requests through the validator that we have to keep a local copy on a scaling group of servers. On occasion, the W3C will update their validator, and it takes time for us to get a copy functioning on our servers. Updating the validator isn’t an automatable task, and can take anywhere from 2-10 hours. This unknown period of time needs to be scheduled in around regular development activities, and thus it can take a week or two after we become aware of a change to be able to implement a new copy on our servers.
Some argue the value of W3C compliance. We advocate compliance wherever possible, however we acknowledge that it is not necessarily a flawless measure of code quality (it is, however, by far the most accepted). Of course you can always exclude this test.