Ordering differs between strings and URIs?

The following query produces the same ordering whether I sort between ?suffix or ?uri on the Virtuoso SPARQL endpoint http://hitontology.eu/sparql:

SELECT (REPLACE(STR(?uri),"http://dbpedia.org/resource/","") AS ?suffix) (STR(SAMPLE(?label)) AS ?label)
{
 ?uri a hito:OperatingSystem ;
      rdfs:label ?label.    
 FILTER(LANGMATCHES(LANG(?label),"en")||LANGMATCHES(LANG(?label),""))
}
GROUP BY ?uri
Case 1: ORDER BY ASC(?uri)
Case 2: ORDER BY ASC(?suffix)

However when I perform the same query using RDFLib and when ordering by ?uri I get a different result:

rows = graph.query(query)
print(list(rows))

[(rdflib.term.Literal('AmigaOS_4'), rdflib.term.Literal('AmigaOS 4')), (rdflib.term.Literal('AmigaOS'), rdflib.term.Literal('AmigaOS')), (rdflib.term.Literal('Android_Ice_Cream_Sandwich'), rdflib.term.Literal('Android Ice Cream Sandwich')),...

In the result above, "AmigaOS_4" is sorted before "AmigaOS", although it should come afterwards.

Strangely, this problem goes away when I add ?uri to the SELECT clause:

SELECT ?uri (REPLACE(STR(?uri),"http://dbpedia.org/resource/","") AS ?suffix) (STR(SAMPLE(?label)) AS ?label)
{
 ?uri a hito:OperatingSystem ;
      rdfs:label ?label.
 FILTER(LANGMATCHES(LANG(?label),"en")||LANGMATCHES(LANG(?label),""))
}
GROUP BY ?uri
ORDER BY ASC(?uri)

[(rdflib.term.URIRef('http://dbpedia.org/resource/AmigaOS'), rdflib.term.Literal('AmigaOS'), rdflib.term.Literal('AmigaOS')), (rdflib.term.URIRef('http://dbpedia.org/resource/AmigaOS_4'), rdflib.term.Literal('AmigaOS_4'), rdflib.term.Literal('AmigaOS 4')), (rdflib.term.URIRef('http://dbpedia.org/resource/Android_(operating_system)'),...

Where does this problem originate? Does it treat "rows" as a set with undefined ordering and when converting to a list it uses some RDFLib internal sorting mechanism?

According to https://www.w3.org/TR/sparql11-query/#modOrderBy:

The "<" operator (see the Operator Mapping and 17.3.1 Operator Extensibility) defines the relative order of pairs of numerics, simple literals, xsd:strings, xsd:booleans and xsd:dateTimes. Pairs of IRIs are ordered by comparing them as simple literals.

RDFLib / rdflib

Ordering differs between strings and URIs? #1466