Yu haven't shown a full stylesheet so we have to guess.

<xsl:template match="a/@*">
   <aa>
     <xsl:copy/>
   </aa>
</xsl:template>


If you apply templates to an attribute of a you would get an aa element
for each attribute, each of which had a copy of the element.
the default templates do not apply templates to attributes, so unless
you have a template that you have not shown which applies templates to
the attributes of a, this template will not be invoked.

<xsl:template match="a">
   <xsl:for-each select="@*">
     <aa>
       <xsl:copy/>
     </aa>
   </xsl:for-each>
</xsl:template>

This matches on a, then works as you showed, generating <aa id="1"/>


david

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