A King Chest can give 2 Legendaries to choose from as it is draft style. Opening it in Arena 3 will have it give 0 Legendaries while opening it in Arena 4 it hasa change to give a choice between Miner and Lava Hound as those are the only Legendaries in Arena 4. And that is probably why entering Arena 4 gives access to 2 Legendaries rather than one.
However there is a workaround for that. If you have to open a Legendary Chest and simultaneously be in Arena 3 or lower, you will have only 1 Legendary card as the Legendary card pool for chests. Then what happens if you open a King Chest. Will it guarantee to never give a Legendary? Will it give you the choice between the same card? Will something else happen?
Worship the 9 Titans
(Golem, PEKKA, Lava Hound, Dragon, Giant Skeleton, Scorcher, Super PEKKA, Electro Dragon, Goblin Giant)
Obey the 4 Immortal Armies
(Skeleton Army, Minion Horde, Goblin Gang, Critter Swarm)
Teach the 12 Titan Babies
(Golemite, Mini PEKKA, Baby Dragon, Lava Pup, Sparky, Inferno Dragon, Ice Golem, Balloon, Cannon Cart, Dropship, Flying Machine, Deployable Turret)
Protect the 12 Peasants
(Skeletons, Goblins, Spear Goblins, Minions, Bats, Critters, Fire Spirits, Ice Spirits, Crystal Critters, Critter Bats, Explosive Drones, Beta Minion)
Babysit the Titan Grandchild
Check my AMA here
This is my favorite discussion ever (not created by me): https://communityroyale.com/discussion/1676/how-arena-1-has-mirror-0-max-trophy#latest