There are similar problems with uploading to publishers in ePub format. The last time I was bashing my head against ebook publishing, about a couple of years ago, many (most? all?) of the sites were validating ePub uploads using an old version of the ePub suite which rejected some ebooks which were valid per the up-to-date validator. Which version they were using was ofc not documented, and you were lucky to even get to see an error message. And of course tech support was largely unhelpful. (Especially kobo.com 's.) The people working on the ePub spec seemed to be largely unbothered by the fragmentation/noncompliance and hideous experience for those authoring and uploading in the format, too.
Which is a pity, because aside from this and some other bugs and pitfalls EPUB 2.0 has some attractive features and is nice to work with for anyone who doesn't mind bashing out a good old directory tree of HTML docs by hand.