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The history of Tetris randomizers: https://simon.lc/the-history-of-tetris-randomizers https://www.metafilter.com/182935/let-piece-I-J-L-TMathfloorMathrandom-4

Truly uniformly random distributions tend to be more clustered than people expect (having runs of the same piece or of mostly the same piece). So later versions took measures to make the piece distribution less random and more non-clustered.