BINDing to inlined data provided by VALUES, and braces

I tried the following query on WDQS:

SELECT ?item ?itemLabel ?string ?StringLabel ?iri ?iriLabel 
WHERE {
  VALUES ?item { wd:Q1339 }
  BIND( STR(?item) AS ?string ).
  BIND( IRI(?string) AS ?iri ).
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}

And the result has ?string and ?iri value. However, if I place an extra pair of braces in the query expression

SELECT ?item ?itemLabel ?string ?StringLabel ?iri ?iriLabel 
WHERE {
  VALUES ?item { wd:Q1339 }
  {
    BIND( STR(?item) AS ?string ).
    BIND( IRI(?string) AS ?iri ).
    SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
  }
}

?string and ?iri in the result are empty, looking as if ?item is missing in the first BIND expression. Why is the result different?

SPARQL's "bottom-up evaluation" is often better understood by its other label, i.e., "inside-out evaluation". That is, nesting is evaluated from the innermost to the outermost.

If you flip the nesting, you'll see the results you expected --

SELECT ?item ?itemLabel ?string ?StringLabel ?iri ?iriLabel 
WHERE 
  {
    BIND( STR(?item) AS ?string ).
    BIND( IRI(?string) AS ?iri ).
    SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
    {
      VALUES ?item { wd:Q1339 }
    }
  }