Many sources (including VNH Poker, pokerstars.com, and cardplayer.com) explicitly list Royal Flush as the highest poker hand.
This seems unnecessary - it's the highest hand anyway as the highest Straight Flush. When designing a ruleset, unnecessary is harmful (e.g. it gives new players more to remember).
So is there a good historical or game-mechanics reason why we treat the Royal Flush as a special case?