That would make every single KO a fluke since many more punches are thrown then end in knockouts.
Well... no. Because chance/probability can vary and accumulate.
So, for instance, the probability of any one shot, thrown by any given fighter, in any given fight, landing and ending in a KO is extremely low, as you have noted (maybe 1 in 300-400 or so?).
But the probability of one shot by a particular fighter (Mark Hunt or Rumble, for instance) landing and resulting is a KO could be more like 1 in 50.
And the probability of a KO shot landing at some point, any point, during a given fight, any given fight, is maybe 1 in 2-3 or so, precisely because the numbers of punches thrown in a fight often enter the hundreds, and with each punch thrown the chance that one of them (any one, not one in particular) will land and end in a KO is increased.
All of which is to say that when a trained fighter who gets a KO every second fight scores a KO against a trained fighter who gets KOd every third fight, that really is a LOT less flukey than when a trained fighter who scores a KO every 20th fight scores one against a trained fighter who gets KOd every 20th fight.
Hall, however, does score his share of dynamic KOs, so the example on hand isn't particularly flukey... but that doesn't mean there is no such thing as a flukey KO.