meaning that it if NPC has 7 cards, you're rolling for 7 card separately in order, so getting no card is quite unlucky. Also, player droprate is working correctly for chests just like NPCs, as the "player" is the one who kills the chest.
you can check our parties. chest of b2 has very often no card in it
And rolling in order would also mean that card #7 is very very rare one o.o
I hope you will change soon something for multiple cards in a single chest
Greetings