Harry's answer is right. The specific flights have different lengths - it's a matter of duty time and the time of day/night, plus other factors like rest pattern and time zones. The night time Osaka and Cape Town flights use them despite being about 10h30 and 9h30 respectively (and the Osaka outbound takes the same augmenter out despite being 9ish hrs in the day time). The FTL scheme gives a max FDP of 15hr for a three pilot crew and 22hr for four man, though I can't see how to extend the max FDP beyond 18hr with inflight rest (I'm missing something in my interpretation, it seems)