XSLT: Trouble combining elements from different nodes in one xsl:for-each loop

I need to transform something like the following (snippet from a FileMaker FMPXMLRESULT export)

<ROW>
   <COL>
      <DATA>A</DATA>
      <DATA>B</DATA>
      <DATA>C</DATA>
   </COL>
   <COL>
      <DATA>1</DATA>
      <DATA>2</DATA>
      <DATA>3</DATA>
   </COL>
</ROW>

into an xls.file, where the corresponding DATA elements are put in separate cells next to each other.

Using the following code

<xsl:for-each select="FMP:COL[1]/FMP:DATA">
    <xsl:variable name="p"><xsl:value-of select="position()"/></xsl:variable>
    <Cell>
        <Data ss:Type="String">
            <xsl:value-of select="."/>
        </Data>
    </Cell>
    <Cell>
        <Data ss:Type="String">
            <xsl:value-of select="../../FMP:COL[2]/FMP:DATA[$p]"/>
        </Data>
    </Cell>
</xsl:for-each>

only returns

A 1

B 1

C 1

instead

A 1

B 2

C 3

and I don't know why...

Any help is highly appreciated!

best Florian