The Myst card shouldn't be added to your inventory (or show up during the first reading), the cards is meant as a "blank" for when there are no more cards of that rarity & tier. So that one is a bug.
The rarity (colored orb) determines how hard to get the cards is. The tier/level increases the "power" of the cards.
So for each tier (1-3) you have cards of different rarities, with tier 3, legendary being the most "powerful" (give or take).
Thank you so much for the postive feedback, I appreciate it! I had almost given up hope, seeing some of these messages here