Please correct me if I'm wrong. According to their respective websites, PCGS states only the lower bell lines need to be unbroken to qualify for the FBL designation. NGC states the top and bottom lines need to be unbroken to qualify for the FBL designation. Peace.
Edited: NGC states all upper and lower bell lines are required for the FBL designation where as PCGS only requires the lower lines. The lower lines are more difficult so that's what PCGS go by. NGC says they have a higher standard.