The 5e of D&D: its like a more balanced version of 2e, but with the character customization frills of 3e and 4e. I love it!
I doubt it's a mistake. I'd bet it's a reaction to the constant complaints that double weapons are over-powered.
Its 100% a mistake. They just copied the damage from the 3.5 double scimitar, and forgot that they upped the scimitars damage dice in 4e.
Always a GM, never a player (not really but sometimes feels like it).