The second example for fn:number() (section 14.4.1) is:

fn:number($item2) returns NaN.

Now $item2 (scroll up to the start of section 14) is bound to an element of the form

  <line-item>
    <description> ... </description>
    <price> ... </price>
    <quantity>5.0</quantity>
      ...
  </line-item>

Since we are told that quantity is typed as xs:decimal, we must infer that this element is schema-validated, and it therefore seems highly likely that the line-item element would be defined with element-only content. In this situation I believe number() should fail with a type error; because although number() itself is resilient to casting errors, the atomization of the argument is done under the function calling rules and will fail in the case of element-only content.

I think it would be better to use a different example, say number($item1/description) (having given this element some non-numeric content).