My understanding is that HMRC believe that in many cases FDP is used as an addition to salary rather than for away from home expenses and thus is fair game for taxation. Thus (eg) employment in short haul charter has minimal opportunity to spend away from home (out n back, short turnrounds), whereas a job in a different segment of aviation may have frequent stops away where FDP needs to be spent.
HMRC have carried out expenditure trials at various airlines and amended the taxation rates depending on the results.
Hope this helps