I think you are pretty close there with the age = SP gained = value.
I've noticed this before when buying players at the top of there Mkt value for their level and age.
The 18yo MC 5* 44 I bought last season his Mkt value was 9.2m, I had him for maybe 10 days.
A: his gains were that of a 23-25yo
B: SP gained made no difference to his value.
C: When he hit 6* his value increased to 9.5m but then stayed at that until the close of the season.
This season back to 5* level higher.
A: Gains have almost doubled so SP gained run in tandem.
B: Value goes up daily by 40K per SP.
Also I bought 4 scouts this season, two on Day one, paid 8.5 & 8.6m
To date they have gained around 14 SP no increase in Value except when I allocated the extra SP to make one 7*, his increased by 200k and not moved since.
On the other hand a 25yo MC who was scout last season and the one before, is flying for his age gaining 120% Avg. and he gets 40K increase for each SP.
So two seasons ago he made 7* back to 6* and now 5* taking him back to the level or maybe one less where he came from when I bought him now has him chasing the last SP to maximise his potential before the rot sets in.
So many links in the chain regarding gains but I do think that players are allocated XXXX gains per season as to keep their values and progression in tandem with the level he's at.
Some of the old forum members may be able to help on this one.
Did justdome not say that he had mainly 4* players and if I remember correctly he didnt get promoted as often as others.
Basically hanging around in a level where your players are gaining at their fastest level and why he only bought 18/19yo in general.