Helicon Books epub validator is more than just another epubcheck. From the website:
Helicon Books validator has many advantages over other EPUB validators. It uses the epubcheck software from the IDPF but adds many additional tests.
First of all we don’t just take epubcheck output and display it, we display an easier to read and easier to understand error messages. Some error messages have long text, we display only the important part and let you read the rest only if you want (by clicking a ‘read more’ link).
Contact us if you need more information about any error, or if you want to learn more about EPUB3 and how to make it better.