I have a totally different experience. I trained team rebounding for two weeks and 7 of my 12 players , ranging from 18 to 25, popped. The trainer was 10. Before that I had been training mostly guard skills and/or shooting skills.
Yes the skills maintain the same training amount regardless of what you train. The skill levels shown do not reflect the sublevels that are truly there. And vice versa, the sublevels gained from training one week are not shown in the stats (although I'm not sure if they are reflected in the game play either).