DMI is calculated based on the actual player skills AND game shape. Game shape tends to have a dominate effect.
Player skills have a fractional component, so that a skill of 5.2 and 5.9 are both called "mediocre". When you train a player, the skill increases, until eventually it will become greater than 6.0 and the skill will be called "average" and you will see a little green arrow. The player still improved his skill, even without the actual visual report. And the DMI could show this improvement, but since there are so many factors, it is almost impossible to isolate the individual ones.
Imagine if you calculated an index based on multiplying your height, weight, and IQ together.
So a tall heavy person of average IQ might have a similar index to a tall skinny genius. Now imagine that you adjusted this by the person's mood. You might not be able to tell whether someone has put on weight, grown taller, or is simply happier.