A stylesheet oddity

I've have come across a puzzling puzzle with the current docbook 5 HTML 
stylesheets. Both the 1.72.0 version and the snapshot from April 29th.

Whilst attempting to change the handling of the d:part template in 
division.xsl I copied the d:part template into my version and the html 
output changed without changing anything in the template.

I have produced a small test case which you can find here 
http://www.sffjunkie.co.uk/docbook/part-bug.zip (it's only 3k in size). 
It uses xsltproc and a small batch file on Windows.

If you run it twice, with and without commenting out the d:part template 
in stylesheet.xsl you can see the difference.

Can anybody shed some light on what is happening 'cause I'm confused.

Many Thanks
Simon Kennedy