For the other training types, players will improve more quickly not only with additional playing time, but also a more skilled player will improve more quickly. For example, a great inside defender and rebounder will find it easier to improve his shot blocking than a poor inside defender and poor rebounder would. This encourages you to develop players with a combination of skills, but there are many combinations of skills which work well together -- you can develop players who will be a good fit for your team's needs.
What this paragraph is referring to is that if you have a MonoSkilled monster say an IS monster and you continue to train IS, you will get diminishing returns as that skill is so far ahead of the others and will be pulled back by the other skills. If you have that same player who's lowest skill happens to be ID, if you chose to train ID it would experience the Inverse effect and actually gain a small bonus to training as the other skills pull it up. This effect pulling a skill down or up is small and extremely hard to measure however it does exist in the game and is built in to encourage balanced players instead of monoskilled monsters.