[Tracker] no "nfo:belongsToContainer" property for some files

["e. Ift" <eiro1980 gmail com>]

hi all,
If all images (nfo:Image) have a parent folder why in the result of the sparql query,

SELECT nie:url(?u) nie:url(?f) WHERE {
  ?u a nfo:Image .
  ?u nfo:belongsToContainer ?f .
}

nothing is returned for nie:url(?f) for some files (images)?