Who knows how the OOTP development gods think...
But if I had to guess it would be that part of the process is:
(1) the gods randomly select something like 1 out of 30 players per period to be visited. More players in the league --> more visits from the gods. The rest of the players in the league are permitted to live their lives in peace and quiet (for the time being).
(2) For those players who get visits from the gods, a random improvement/hit is given to them. The gods determine the size of the effect on each of these players by picking a random number from some symmetrical distribution with a mean of zero.
(3) Therefore, no matter how many players exist on a team or are in a league, the expected average development change will always be zero. This is the easiest way for a programer to model visits by development gods and, in addition, it means that the average talent rating of the league is (simplifying here) is unaffected by the number of players in the league.
Lots of mathematical/statistical nuances are ignored above, but the above would be my guess about one part of what goes on in the heads of the development gods.
(May the gods be praised and may my players be deemed worthy to get nice visits from the gods.)
