[xsl] Sorting seems not to work completely

["Mark" <mark@xxxxxxxxxxxx>]

I cannot get the @value to sort in descending order. My XML (simplified) and 
template are below. The sorts on the Date attributes work, but the one on 
the value attribute does not.  Please, what am I doing wrong?
Mark

<Item>
   <Stamp scott-number="2888" pofis-number="12" value="1" 
type="definitive">
     <Date year="1993" month="7" day="1"></Date>
     <Inscription czech="Ceski Budejovice" eng="Ceske 
Budejovice"></Inscription>
     <Theme>Cities</Theme>
   </Stamp>
</Item>


<xsl:template match="List">
   <xsl:element name="List">
    <xsl:for-each-group select="Item/Stamp" group-by="concat(Date/@year, ' 
', Date/@month, ' ', Date/@day)">
      <xsl:sort select="Date/@year" data-type="number"></xsl:sort>
      <xsl:sort select="Date/@month" data-type="number"></xsl:sort>
      <xsl:sort select="Date/@day" data-type="number"></xsl:sort>
      <xsl:sort select="@value" order="descending" 
data-type="number"></xsl:sort>
      <xsl:element name="Item">
        <xsl:copy-of select="current()/Date" 
copy-namespaces="no"></xsl:copy-of>
        <xsl:copy-of select="current-group()" 
copy-namespaces="no"></xsl:copy-of>
      </xsl:element>
      </xsl:for-each-group>
   </xsl:element>
 </xsl:template>