xsl loop with variable

Hi all:
I have the next xsl code:

  <xsl:variable name="numMonth" select="0"/>
  <xsl:variable name="lastMonth" select="00"/>

  <xsl:for-each select="listObject/object">

        <xsl:if test="not($lastMonth=substring(editionDate,4,2))">
          <xsl:variable name="numMonth" select="$numMonth+1"/>
        </xsl:if>
        
        Num months:<xsl:value-of select="$numMonth"/>

        <xsl:variable name="lastMonth" select="substring(editionDate,4,2)"/>

    </xsl:for-each>

I can't make "numMonth" variable update with the correct increment out of the 
"if" tag, so that "Num months:" show me the correct value (until now only show 
'0', the original value).
How can i do that? Can anybody help me please?

Thanks.
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