> The resulting xml file I am getting is:
of course without showing us the input file it;'s a bit hard to guess...
but 

????? <xsl:copy>
that makes a tr node

? <xsl:for-each select="td">
??? <xsl:attribute name="class"

that (repeadedly) adds a class attribute to this tr node once for each
td in the source.

You presumably want something like

<xsl:template match="tr[td[not(a) and text() != 'Data not available' and not(@*)]]">
????? <xsl:copy>
?? <xsl:choose>
?? <xsl:when test="position() mod 2 = 0">
?? <xsl:for-each select="td">
    <td class="evenRowStyle">
???  <xsl:apply-templates select="node()|@*"/>
    </td>
   </xsl:for-each>
?? </xsl:when>
?? <xsl:otherwise>
?? <xsl:for-each select="td">
     <td  class="oddRowStyle">
???  <xsl:apply-templates select="node()|@*"/>
    </td>
   </xsl:for-each>
?? </xsl:otherwise>
?? </xsl:choose>
???? </xsl:copy>
</xsl:template>

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