I can assume that this highly efficient demon harvesting happens in a simple and understandable scenario: imps, gargoyles, and hounds, after a slight boost, gain extra hit points, and maybe even a ring, and by the 3rd day, when you can start harvesting demons, the untrained creatures are gradually replaced with trained ones, and thus, by the time you need to harvest the starting creatures, they are already fatter, resulting in higher hit points than what Vicc prescribed.

This is essentially what experience gives you in this case, especially imps get quite fat – with +1 hit point, they gain 25%!