I can assume that this highly efficient demon farming happens in a simple and understandable scenario: imps, gargoyles, and dogs, after a slight boost, gain extra hit points, and maybe even a ring, and by day 3, when you can start farming demons, the untrained units are gradually discarded, while the trained ones are leveled up, and thus, by the time you need to discard the starting units, they are already stronger, resulting in higher hit points than what Vicc prescribed.

This is essentially what experience gives you in this case, especially imps gain a good amount of hit points – with +1 hit point, they gain 25%!