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It's my head canon that the Elements of Harmony earned their element by exemplifying it, but their power is invoking that trait in others. So Dash doesn't have to be loyal and Applejack can lie but with a supernatural tendancy you'll always find that,
1) Everyone is on Rainbow Dash's team
2) Nobody can lie to Applejack
3) Being mean to Fluttershy is unheard-of
4) Good luck not laughing when Pinkie is around
5) Everyone bends over backwards for Rarity
6) Everyone is friends with Twilight Sparkle
So, ideally Rainbow Dash wouldn't be the most loyal pony, but the kind of pony you'd want to be loyal to.
That said, the real reason AJ and RD are swapped is because after you claim someone is for instance the most honest pony in the land, it's funny to make them lie. It's just attempted comedy from the 4th wall.