Opening just 5 chests is a horrid drop chance and a horrid way of deciding whether to continue. That is like putting a single coin in a slot machine and hoping you get the jackpot.
Actually, the algorithm is thus apparently. Take the drop table, multiply each drop by the weight of the item and add to an array (a list), at the end randomize the list. Now do this for all drop tables that have an expiration date past 6 days (ie new event) and you are done. (Atlas chests and badge chests only randomize every new Atlas season.) Instead of the automatic randomization, it could be part of the event/season startup sequence too.
So how many platinum chests would you have to open to see the sequence?
7570 Epic, 25 Legendary, and 15 Mythic in the main sequence. Sorry I was wrong, it is 5 mythic per 100 not 1. I misread my own data.
5 Legendary, and 1 Mythic in the bonus sequence.
Epic count is 39.
Legendary count is 38.
Mythic count is 32.
You would get 29 mythic drops on average every 180 chests opened. So to get the entire mythic sequence, you would need to open 2,880640 platinum chests. By that point you would have gotten so many multiples of even the legendary it would have evened out the huge mythic drop.
To top it off, if you were wanting sigils, your chance of getting sigils is 8/39 epic prizes, 5/38 legendary prizes, and 5/32 mythic prizes.
So really, if you want lots of goodies from platinum chests, you need to open a lot. Even 50 will get you much closer to the average than just 5 will.
Edit: FYI, the drop tables are not in the game files. You have to grab it separately every event. It’s not just downloaded and stored to your phone like the tower/dragon info is.