Find A Grave policy is that the dates on the page must match the gravestone, when shown. The difference to the actual can be explained on the page, but they are supposed to use the gravestone dates, when available, even if they do not match sources.
Death Certificates can have an incorrect Birth date / Location, etc because the person giving the information was incorrect. That doesn't mean it shouldn't be used.
People, especially females, frequently used incorrect Birth information.