Having you on SBY for a period after immediately following an FDP is perfectly legal, so long as the total duty time is taken again as rest. It can be a useful period for crew schedullers when they know they are tight on next day cover. Having someone on SBY following an FDP gives them an extra couple of hrs to look forward and re-assign a duty the next day and thus removing the worry of trying to make contact with someone who has finished an FDP and who has switched off the phone !!