I believe FEW is only 1-2 octas, SCT is 3-4, BKN is 5-7, and OVC is 8. So the ceilometer is scanning the sky in every direction/elev angle looking for clouds. So you could have an OVC layer overhead and north of the station with the south side clear and it may get reported as BKN even though there's no drop in insolation. You could also be FEW/SCT and have one Cu sitting right in front of the sun too. Then there's the CLR debacle where many ASOS ceilometers only report up to 12kft so in advance of systems you can get half a day's worth of CLR obs despite an OVC altostratus layer at 15kft and then suddenly it lowers enough and you go from CLR to OVC120 (12kft).