[xsl] Difficulty over understanding preceding-sibling

["Joe Fawcett" <joefawcett@xxxxxxxxxxx>]

Can anyone explain why the first item being passed to the template has any 
preceding-sibling?
XML:
<root>
 <item id="1" />
 <item id="2" />
 <item id="3" />
 <item id="4" />
 <item id="5" />
 <item id="6" />
 <item id="7" />
 <item id="8" />
 <item id="9" />
 <item id="10" />
</root>

XSLT:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
 <xsl:param name="groupSize" select="3" />
 <xsl:template match="/">
   <new>
     <xsl:call-template name="groupItems">
       <xsl:with-param name="items" select="root/item" />
     </xsl:call-template>
   </new>
 </xsl:template>

 <xsl:template name="groupItems">
   <xsl:param name="items" />
   <xsl:variable name="itemCount" select="count($items)" />
   <xsl:if test="$itemCount &gt; 0">
     <group>
       <xsl:for-each select="$items">
         <xsl:if test="position() &lt;= $groupSize">
           <newItem id="{@id}">
             <xsl:if test="position() = 1">
               <xsl:attribute name="preceding-sibling-count">
                 <xsl:value-of select="count(preceding-sibling::item)" />
               </xsl:attribute>
             </xsl:if>
           </newItem>
         </xsl:if>
       </xsl:for-each>
     </group>
     <xsl:call-template name="groupItems">
       <xsl:with-param name="items" select="$items[position() &gt; $groupSize]" 
/>
     </xsl:call-template>
   </xsl:if>
 </xsl:template>
</xsl:stylesheet>

Result (msxml version 4):
<new>
<group itemCount="10">
 <newItem id="1" preceding-sibling-count="0"/>
 <newItem id="2"/>
 <newItem id="3"/>
</group>
<group itemCount="7">
 <newItem id="4" preceding-sibling-count="3"/>
 <newItem id="5"/>
 <newItem id="6"/>
</group>
<group itemCount="4">
 <newItem id="7" preceding-sibling-count="6"/>
 <newItem id="8"/>
 <newItem id="9"/>
</group>
<group itemCount="1">
 <newItem id="10" preceding-sibling-count="9"/>
</group>
</new>

Why doesn't the preceding-sibling-count show zero, or am I just up too late?

--

Joe