PHP xpath not able to find child node but finds parent

I am able to load an XML document and create a SimpleXMLElement - I am also able to query it using XPATH but I am perplexed by the behavior I am noticing. When I load the XML and create a SimpleXMLElement - it is of the below structure

[0] => SimpleXMLElement Object
        (
            [PersonName] => SimpleXMLElement Object
                (
                    [FullName] => Tom Jones
                    [GivenName] => Tom
                    [MiddleName] => SimpleXMLElement Object
                        (
                        )

                    [FamilyName] => Jones
                )

When I execute $xml->xpath('//PersonName') I get the right result which is as below

[0] => SimpleXMLElement Object
        (
            [FullName] => Tom Jones
            [GivenName] => Tom
            [MiddleName] => SimpleXMLElement Object
                (
                )

            [FamilyName] => Jones
        )

But when I execute $xml->xpath('//GivenName') I get the below result - which I am unable to understand

[0] => SimpleXMLElement Object
        (
        )

Can someone please help me understand what my error is?