As you've mentioned I can guess whose got the Saturdays Off.....probably the same old Postie Mafia that nab all the decent duties and have first choice for Holidays!
In a way I could understand a manager who gave more perks to say a hardworking reserve (who covered loads of duties and worked his day off etc.....not saying I agree with that), but it's strange that they just seem to go out of their way to please a few of the Old & Bold, who would never do anything more than they can get away (and often a lot less) and wouldn't dream of helping any of their colleagues out!