Xquery: same test has different result whether used in switch/case or in if/then/else

I can't find an explanation on the following. I've made this test script in order to solve my previous question.

xquery version "3.0" ;
declare default function namespace 'local' ;

declare function local:is-img-only( $element as element() )  as xs:boolean {    
   ($element/child::*[1] instance of element(img)) 
   and (fn:not($element/child::*[2])) 
   and (fn:normalize-space($element) = '')
} ;

let $in-xml := <xml>
   <p id="1">

      <img id="1"/>


    </p>
    <p id="2">
      <img id="1"/>
      hello
    </p>
    <p id="3">
      <img id="1"/>
    </p>
    <p id="4">
    <blockquote>hello</blockquote>
      <img id="1"/>
    </p>
    <p id="5">
      <img id="1"/>
      <img id="2"/>
    </p>
</xml>

Then, the following using if then else :

for $p in $in-xml/p
  return if (local:is-img-only($p)) 
    then $p/@id/fn:data() || ' has only an img child'
    else $p/@id/fn:data() || ' has not strictly an img child'

returns as expected :

1 has only an img child
2 has not strictly an img child
3 has only an img child
4 has not strictly an img child
5 has not strictly an img child  

Whereas the following using switch case

for $p in $in-xml/p
  return switch ($p)
    case (local:is-img-only($p)) return $p/@id/fn:data() || ' has only an img child'
    default return $p/@id/fn:data() || ' has not strictly an img child'

returns as not expected :

1 has not strictly an img child
2 has not strictly an img child
3 has not strictly an img child
4 has not strictly an img child
5 has not strictly an img child

Any explanation ? Why Conditional Expressions would not behave the same way than Switch Expressions ?