yer i guess when i say better, i mean to say, you get better training value for one individual player.
There are quite a few good threads and info on this topic, check out the help section. But in summary, say you train Inside shot at one position (centre) compared to two positions (C and PF), you get more sub-level pops using one position training, compared to two position (but with two position, you can train twice as many guys).
So using one position training, your centre might have Respectable Inside shooting, which is equal to a 7.0 After one position training, he will be (dependant upon training minutes and height) a 7.8 level of inside shooting (this is what i mean by sub-levels.)
Using two position training, that same player, with same training minutes, may only go to say, 7.4 level of inside shooting.
In other words, you would have to train maybe 3-4 weeks of inside shot, on that one player using two position training, to get one 'pop'.... compared to one position training, which could pop 1-2 weeks.
Read this thread for more info on this
Training Basics
(144856.30)and do some searching on sub-levels of the skill set.