How does the lootrun chest really work?

Heya, Ive been doing a bunch of lootrunning lastly and Ive managed to get
my hands on some mythics. 4 in total since the lootrun update. Ive found
all of them in the sky Islands expedition (Cause its the only one I can actually
do more than 20 challenges xD). But what I find interesting is that Ive found
all of them in regular chests or challenge reward chests (This includes flying chests).
Ive done far more than 2500 pulls on the end reward chest and have never
gotten any mythics. Not even the corkian augments. Its really weird to me
because everyone seems to get their mythics from the end reward chest while
I have never got one from the pull. So is there a way to maximize the chances of
getting a mythic from the end rewards?

 

whats your build?

 

Nothing that contains a mythic weapon. Im lootrunning with mage and cant afford a good mythic wand.
Event horizon
Staff of the legends for easier travel
Pisces
Cardiac Arrest
Seipodon
Stardew
Photon
Summa
Subur Clip
Ensa's Faith

 

Besides just getting a bunch of rolls and rerolls, there's nothing you can do to buff the end rewards chest iirc