My initial thought is that yeast can't go on multiplying (dividing themselves) over and over despite addition of fresh wort, about 3-4 splits and they've depleted their initial reserve of 'nutrients' and should be entering the stationary phase and settling out rather than being force fed sugars from additional wort pitches (a bit like under pitching)
I'm sure someone far more learned will weigh in soon with the science.
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