The underlying cause of this variability is that many semiconductor devices are highly sensitive to uncontrollable random variances in the process.

Adding Predictors to the Model: It is possible that some of the random variance (i.e. variance associated with individual differences) may be attributed to fixed predictors other than time.

The purpose of the random variance is to find close to globally optimal solutions rather than simply locally optimal ones, the idea being that the random element decreases as the algorithm settles down to a solution.