@burcinc-- If the appraisal is based on quantitative indicators, then the same employee will be rated the same way regardless of who evaluates him. So for example, if the indicator is the number of gadgets a worker puts together in one hour, the evaluation will be the same across the board.
If however the appraisal is based on qualitative indicators, then the rating can certainly vary from one evaluator to another. This type of rating not only depends on the aspects of performance the evaluator is asked to rate, it also depends on the evaluator's methods and opinions.
It is possible to regulate this by setting specific objectives and by using pre-determined scales for the evaluator to simply fill out. But if the evaluator is using an essay type appraisal method, the results can come out vastly different because there is too much leeway about how this appraisal is done.