The FrameDifference class is not implemented correctly

Looking at the FrameDifference class there is a method called select_annotated_frames: https://gitlab.idiap.ch/bob/bob.pad.face/blob/90ad3d5ace98113407a310410f916313f44d2fc6/bob/pad/face/preprocessor/FrameDifference.py#L315

This method removes the frames which are not annotated. However, this breaks the continuity of frames which this algorithm depends on. Am I missing something here? Aren't the frames supposed to be continuous because the algorithm works on current and previous frames?