Steps to reproduce:
-Create a space
-Go to Administer Space and return to WebHome of the space.
-Go to panel list and drag the Orphaned pages panel.
You will see there that the WebPreferences page of your created space is orphan.
NOTE: This happens to newly created space. The ones in default bundle have as parent their Spacename.WebHome page.
This is not a bug, but it should be fixed because it can create inconsistencies.