margin-left not correct on printed output

Hi guys

I've got the following FO code.

<?xml version="1.0" encoding="UTF-8"?>
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">

    <fo:layout-master-set>
        <fo:simple-page-master master-name="Letter" page-height="297mm"

page-width="210mm"

margin-left="25mm"

margin-right="25mm" >
            <fo:region-body region-name="body" />
        </fo:simple-page-master>
    </fo:layout-master-set>

    <fo:page-sequence master-reference="Letter">
        <fo:flow flow-name="body">
            <fo:block>Should be 25mm from left edge of paper.</fo:block>
        </fo:flow>
    </fo:page-sequence>
</fo:root>

When printed using Acrobat the resulting printout does not have a 25mm
margin. I've tried various margin widths and only 0mm works as expected.

How does this work? Am I doing something wrong? I've also tried various
adjustment settings from the Adobe Acrobat print dialog with no success.

Thanks
Gary