What you should be experiencing is that the sitemap order is based on the first child page being listed on after another. For example, your sitemap order might be:

First Page
First Page First Child
First Page Second Child

Second Page
Second Page Child

Third Page
Third Page Child

The page list sitemap order orders them in the following way:


First Page
Second Page
Third Page
First Page First Child
Second Page Child
Third Page Child

I believe this is the case. We happen to have a project where we output only the children of the top pages and instead of the sitemap order listing all of the Children under the First Page, then moving down the sitemap tree, the pages were listed as follows:

First Page First Child
Second Page Child
Third Page Child
First Page Second Child

I hope that helps.