Documentation for extraAllowedContent is confusing, what does it do?

Regarding http://docs.ckeditor.com/#!/api/CKEDITOR.config-cfg-extraAllowedContent

It there defines rule like 

extraAllowedContent: 'b i',

And then after a few lines

editor.filter.check( 'b' ); // -> true (thanks to extraAllowedContent)
editor.setData( '<h1><i>Foo</i></h1><p class="left"><b>Bar</b> <a href="http://foo.bar">foo</a></p>' );
// Editor contents will be:
'<h1>Foo</h1><p><b>Bar</b> foo</p>'

This to me is very confusing. It saves the <b> tag due to extraAlowedContent, but it strips the <i> tag even though it is defined in extraAllowedContent. Is this a problem with the documentation or what does it actually do?