schema: error validation depending on global or local element declaration

Greetings,

I'm getting a validation error depending of the declaration of my
elements are local or global.

For instance, this sample xml:

<test> 	
	<a/> <b/> <a/>
</test> 

is reporting this error:

error:cvc-complex-type.2.4.a: Invalid content starting with element 'a'.
The content must match '((("":a)|("":b)))

The schema is:

   <!--
     <xsd:element name="a" type="aType"/>
     <xsd:element name="b" type="bType"/>
    -->
 <xsd:element name="test">
  <xsd:complexType>
   <xsd:sequence>
      <xsd:choice>
      <!--
        <xsd:element ref="a" />
        <xsd:element ref="b" />
      -->
     <xsd:element name="operation" type="aType"/>
     <xsd:element name="transaction" type="bType"/>
      </xsd:choice>
   </xsd:sequence>
  </xsd:complexType>
 </xsd:element>

If I replace my local elements declarations, a and b, and use the
declaration by ref (using the commented code) it works fine.

Why is that happens?

regards,
Pedro Salazar.
-- 
/ps