Variable Scope

Hi,
  I've been looking through the FAQ and have a question about one of the 
examples under the "Basic Troubleshooting/Tables" section. The input is

<?xml version='1.0'?>
<TASKS>
  <TASK>
    <COMPONENTS>
      <COMPONENT>A</COMPONENT>
      <COMPONENT>B</COMPONENT>
      <COMPONENT>C</COMPONENT>
      <COMPONENT>D</COMPONENT>
      <COMPONENT>E</COMPONENT>
      <COMPONENT>F</COMPONENT>
      <COMPONENT>G</COMPONENT>
      <COMPONENT>H</COMPONENT>
      <COMPONENT>I</COMPONENT>
      <COMPONENT>J</COMPONENT>
      <COMPONENT>K</COMPONENT>
    </COMPONENTS>
  </TASK>
</TASKS>


and the XSL includes

<xsl:template match="TASKS/TASK/COMPONENTS">
  <xsl:variable name="t-size" select="count(COMPONENT)"/>
  <xsl:variable name="half" select="ceiling($t-size div 2)"/>

<TABLE>
  <xsl:for-each select="COMPONENT[position() &lt;= $half]">
   <xsl:variable name="here" select="position()"/>
        <TR>
      <TD><xsl:value-of select="."/><xsl:value-of select="$here"/></TD>
      <TD>
        <xsl:choose>
	  <xsl:when test="../COMPONENT[$here+$half]">
	    <xsl:value-of select="../COMPONENT[$here+$half]"/>
          </xsl:when>
          <xsl:otherwise></xsl:otherwise>
        </xsl:choose>
      </TD>
    </TR>
  </xsl:for-each>
</TABLE>

</xsl:template>

What I'm not clear about is why the 'here' variable is updatable as the 
for-each iterates, it was my understanding that variable values cannot be 
changed. 
Could someone explain ?

Thank you.

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