What I have a hard time figuring out is what is meant by "...and improve them twice as much".
Legendary is not the cap it simply a title one legendary ebony bow can be superior to another. Let me give you an example, my archer, assassin has steel smithing, arcane smithing and elven smithing. My smithing level is 100 and with my crafting equipment its fortified by 125%.
I picked up with Ebony bow awhlie back and had just thrown it in a trunk planning to sell it later. I never got around to it so it will serve as an example.
Without smithing it will deal 51dmg in my hands not counting arrows or other magic boosters, my unmodded elven bow deals only 39dmg.
With 100 smithing alone the Ebony Bow is brought to 70dmg(flawless) and the elven bow to 74(legendary). A Dwarven bow on the other hand goes from 36 to 55(flawless). Do you see now what improve twice as much means?
Now lets say I have my smithing gear on.
Ebony Bow 81dmg Legendary.
Dwarven Bow 66dmg legendary
Elven Bow 101dmg Legendary*because my perk lets me improve it twice as much*