Time, location, attention and need
It is an utopia to think that people perceive objectively. Practical issues such as time and location play a role. If an employee works from home or a different location than the manager on a daily basis, it is extra difficult to see, hear and feel how an employee operates. Attention is also important in perception. If we do not pay attention to a subject, we do not perceive it. During a good movie on TV, it is even less noticeable if your partner has been to the hairdresser. The same principle applies to employees. If the manager is busy doing all kinds of things, the employee who does not ask for attention will not be noticed. A person's needs also play a big role in perception. Just as someone who is hungry sees food everywhere, the manager whose department needs to become more customer-oriented will pay particular attention to the employees who share good examples of this.
Self fulfilling prophecy
We interpret the observations we make in different ways, even explaining facts differently. After the first impression, we mostly perceive facts that confirm the first impression. It is rare that we drastically adjust that image. On top of that comes the halo effect, which ensures that when certain qualities are present we quickly think that other qualities are also present. Both effects are further strengthened by the self fulfilling prophecy. We approach the employee about whom we think positively, with confidence in a good outcome, so that this will also happen much more quickly.
There's no accounting for taste
With these limitations of our perceptions and interpretations, it is not surprising that a new manager will regularly come to completely different perceptions of the quality of employees. We cannot eliminate these differences; we must accept how the human brain works. Just as we accept that there is no accounting for taste. Because no matter how objectively we try to observe, one manager likes Brussels sprouts and another does not.