Use of separator

i have some trouble understanding the separator-attribute of value-of.
This is my template:

<xsl:template match="example">
  <helloWorld>
     <xsl:value-of select="element()/text()" separator=", "/>
  </helloWorld>
</xsl:template>

The element "example" contains several child-nodes with text. The
above expression gives the  expected values but without the separator
("TextTextText"). But if i change the value-of expression to this...

Now i wonder why the result of the first expression contains no
separator while the other one does. Any explanations?