It happens when the laptop has a repair, normally a logic board replacement. When this occurs the Apple Store that carried out the repair has not correctly logged the replacement board with the items serial number so OSX cannot see the serial number. It happens all the time as Apple seem to carry out the repairs quite lapsely quite often.
So basically laptop breaks for whatever reason. You go in store, have it fixed with a logic board replacement. Apple does not log the replacement parts correctly. No serial is then displayed.