[xsl] Can keys go out of scope?

I know keys can't go out of scope, I just want to know why I'm having to do
this:

 <xsl:template match="Billcode">
   <xsl:variable name="current" select="."/>
   <xsl:apply-templates 
     select="$template/sc:static/*" mode="static">
     <xsl:with-param name="current" select="$current"/>
     <xsl:with-param name="score" select="key('scores',@ScoreRef)"/>     #I
shouldn't need to do this
    </xsl:apply-templates>
 </xsl:template>


The key only gets used at the other end of some chained templates, and I
would have hoped to be able to do this:
<xsl:value-of select="key('scores',$current/@ScoreRef)"/>,

given that I'm passing the Billcode element down the chain in the $current
parameter. I've been through this n times, it even works on a different
stylesheet. I've tested the value of $current/@ScoreRef at every step of the
way, and it's what it was in the first template shown above. Any idea what's
going on (or, (@~!#  as Captain Haddock would say)?

Thanks for your help
Tom




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