Hey,
So your math is right and technically you would receive more training by doing this overall. However, nobody uses this training method for a very good reason. Usually, when people get trainees, they get them for a specific purpose (train for NT, make D1, make money, etc). So let's run through some popular options for methods:
NT: A player can't make the national team or U21 receiving only 30% training, so this one is obviously out of the way.
D1: Even if you trained your whole roster at 30%, you'd only reach the skill level of a U21 team at age 27. 10 years x .30 = 3 years of 100% training. A good U21 team is probably D3/D2 in the USA. So D1 is out of the question.
Make Money: While you do get more training, you end up with a bunch of players who aren't that well trained for their age. So you can't really make much money from this method since the players won't be good and young at the same time.
Not to mention that having 9 trainees on a roster is ridiculous and hard to be competitive while doing so...
TLDR: Training at 100% for trainees is the best/most effective method. Keeps trainee size manageable, a potential for NT, D1, and profit