Another commenter (dragontamer) pointed out elsewhere in the thread that Apple might be doing what Sony did for the PS3 (since Sony also made custom chips that had to perform identically in the end product): the strategy Sony took was to actually make better chips than advertised for the PS3, and disable the extra cores. That means that if one of the cores is broken, you can still sell it in a PS3; you were going to disable it anyway. Yields go up since you can handle a broken core, at the cost of some performance for your best-made chips since you disable a core on them.

That could make sense for Apple; the M1 is already ~1 generation ahead of competitors, so axing a bit of performance in favor of higher yields doesn't lose you any customers, but does cut your costs.

Plus, they definitely do some binning already, as mentioned with the 7 vs 8 core GPUs.