Very often, film rolls contain frames that were improperly exposed or too blurry to produce a scannable/printable image. This can be caused by incorrect setting for the film’s ISO, a malfunction of the camera, opening the camera back before the film has been re-wound, accidentally taking a photo when the camera lens cap is on, and several other similar scenarios. These frames are neither scanned nor printed.
Phoenix Photo LabArchival Quality Prints, Digital Imaging and Film Processing