I could be wrong, but it seems to me that the reason for decrease in training has more to do with the number of positions trained rather than the number of people trained. If it was the number of people, then you could do team passing training with just 5 players and get skill pops faster than training SG/PG with 6 players. This wouldn't make sense.
In other management games it seems that training that is limited to less people is faster partly due to the economical and tactical advantage given to each slot. For example, in HT, keepers train fastest, but you can only train a maximum of two players. Defenders train the slowest, but you can train 10 players.
In BB, if you drop your training focus to 1 position, the max that you could fully train would be 3 (given that you only play 1 player in the training slot per match and they don't foul or injure out). Your team's ratings wouldn't increase by that much (tactical handicap) and you wouldn't have as many players to sell (economic handicap).
This is of course, just my guess and I could be completely wrong, but I seriously doubt that training only 2 players in 2 slots (PG/SG combo) would be more effective than training 3 in one slot only.