Reasoning behind epub <h> level numbering

I have encountered the issue of epub and web files having different
    <h> title level numbers for the same content. It follows on
    from Robert Nagle's posting of 15 March 2011. 

    

    Specifically, the epub stylesheets produce titles one level higher
    than the web stylesheets, e.g.:

    

    preface title: web (h2), epub (h1)

    

    This is consistent behaviour for the elements: preface, glossary,
    bibliography, index, sect1 through to sect5, and bridgeheads sect1
    to sect5.

    

    The epub docbook.xsl has modified HTML H level numbering in both
    <xsl:template match="d:bridgehead"> and <xsl:template
    name="component.title"> (apparently as an SEO customisation).

    

    Can someone elucidate why there is this change in <h>
    numbering for the epub customisation?

    
-- 

      Dave Gardiner