I was surprised too, none of this is guessing however. All of this was posted by a BB on the forums and then looked over and approved for posting by a BB as well. So it's been confirmed by a BB not once, but twice.
However, I can go into my interpretation of the details a bit more unofficially (nothing I comment to you is actually official, the only thing I've confirmed being the main post). The 10% is 10% of the training amount, so if you have an older player, or got less minutes, or are training something that trains slowly, that 10% will be much smaller. It doesn't mean the skill gets 0.10 further in progression.
As an 18 year old, training something like 1v1, you'd get the max amount from these crosses. However as the player even gets to 21-22, 10% of that training is about 70% or so of what you'd see at 18 and thus so is every single cross-training as well. That's sort of where it all evens out in my eyes and the gym might play a much larger role in producing the high TSP we see at the U21 level now. We all believed it was mostly in the Youth Trainer, but I think both played a factor in the +15 or so TSP we see after the buffs.
2x NBBA Champion: S55, S56. 3x USA Cup Champion: S54, S55, S56. WR for longest home streak ever at 11 in Season 47.