XT and SAXON treats //ccc and /descendant::ccc differently

I just run to this.

By now I thought that //foo is an abbrev form of /descendant::foo

But running both XT and Saxon I was given the following answer to these:

<xsl:value-of select="//ccc[2]"/>   c12
<xsl:value-of select="//ccc[5]"/>   gives no hit
<xsl:value-of select="/descendant::ccc[5]"/> gives c22  

for following XML file :
<aaa>
<bbb>
<ccc>c11</ccc>
<ccc>c12</ccc>
<ccc>c13</ccc>
</bbb>
<bbb>
<ccc>c21</ccc>
<ccc>c22</ccc>
<ccc>c23</ccc>
</bbb>
</aaa>


Am I missing something or  is there some shared bug ?  Thanks. 

BTW: I am considering to start at Zvon a collection of parser conformity
test files. Is somebody already working on it so I do not reinvent the
wheel?
-- 
******************************************
<firstName> Miloslav </firstName>    
<surname>   Nic      </surname>     

<mail>    nicmila@xxxxxxxxx    </mail>   
<support> http://www.zvon.org  </support>


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