Isn't that what they expect the cloud cover to become by the time the next METAR is issued?
I assumed that the last BKN or OVC would be the general cloud condition above whatever layers were specifically mentioned. So above the first BKN layer there would be a second BKN or OVC layer, but the height of that layer could not be determined, or would be too high to be included in a METAR.
If it were to be a prediction, wouldn't they need to add a "BECMG"?