The mismatch performance derating is mainly due to the fact that in a string of modules (or cells), the lowest current drives the current of the whole string. When installing real modules in the field, the characteristics of each module are never rigorously identical. The characteristics, such as current, voltage and power, can vary from module to module. Long strings are thus driven by the weakest part.
0.4% is a usually accepted value for mismatch loss.