Not to be confused with diminishing relative returns, where the investment gives the same returns each time but the functional result of the stat gives diminishing returns relative to the stat you had prior to each step of investment.
IE if you have "100% increased damage", if you go from 0% increased to 100% increased, thats x2 dmg.
But if you go from 900% increased to 1000% increased, thats only x1.1 dmg