Umbraco V6 404 not handled properly according to culture

Question

I have an umbraco site (umbraco v6), and I have set up custom 404 handling as per http://our.umbraco.org/Documentation/Using-Umbraco/Config-files/umbracoSettings/index. My content tree is as follows:

Content
    Root
        Greek-Homepage
            Greek-Content
            Greek-Not-found (id: 1303)
        English-Homepage
            English-Content
            English-Not-found (id: 1304)

And I've set up the <errors> tag as follows:

        <error404>
        <errorPage culture="default">1303</errorPage>
        <errorPage culture="el-GR">1303</errorPage>
        <errorPage culture="en-US">1304</errorPage>
    </error404>

No matter what I try, I can't get the site to display the 1303 (Greek) node. It only shows the 1304 (English) node.

What am I doing wrong here?