variable going out of scope in a for-each loop?

xsl fragment:

     <xsl:for-each select="cell">
         <xsl:if test="number(.)=number(.)">
           We have a number here 
         <xsl:choose>
           <xsl:when test="position() mod 2 = 0">
             mod 2 = 0!
             <xsl:variable name="aantal_uren">
               <xsl:value-of select="."/>
             </xsl:variable>
             The value of the aantal_uren is: <xsl:value-of select="$aantal_uren"/>||
             The value of the uur_tarief is: <xsl:value-of select="$uur_tarief"/>||
           </xsl:when>
           <xsl:otherwise>
             mod 2 != 0!
             <xsl:variable name="uur_tarief">
               <xsl:value-of select="."/>
             </xsl:variable>
             The value of the aantal_uren is: <xsl:value-of select="$aantal_uren"/>||
             The value of the uur_tarief is: <xsl:value-of select="$uur_tarief"/>||
           </xsl:otherwise>
         </xsl:choose>
         </xsl:if>
         ....

Now, as soon as I loop form the first number containing cell to the next, the 
value defined in the previous-sibling seems to go out of scope viz.
We have a number here 
mod 2 != 0!
The value of the aantal_uren is: ||
The value of the uur_tarief is:
246
||
<fo:table-cell ><fo:block font-size="8pt" text-align="center">
246
Here are the aantal
And the tarief
</fo:block></fo:table-cell>
We have a number here 
mod 2 = 0!
The value of the aantal_uren is:
135
||
The value of the uur_tarief is: ||
<...

why?
-- 
Eric Smith - currently using xalan and xsltproc - what a difference speed makes - and fop on linux

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list