Comment by PetitPrince

Secret grade is done within the confines of the game with no luck manipulation.

Note that the piece distribution algorithm is usually not a pure random() function, but one that's optimized to remove or reduce piece repetitions. For TGM it's history based (keep in memory the 4 last pieces type, reroll in case of repetition, with a 4 or 6 reroll limit depending on the game). IIRC Shuey uses a ruleset that uses the standard 7-bag algorithm (it's a Fisher-Yates with 7 elements; put all 7 types in a collection and deplete them one by one, randomly).