Evaluations should never be a popularity contest. If you can't evaluate based on the job description, then rewrite it until you can. Cloning might work for sheep, but it's dangerous at work. Resist the temptation to reward only people who look and act like you. This robs them of the chance to improve or to be recognized for good work.
Some managers skew high for fear of being critical.
Others skew low because they believe poor ratings encourage improvement. Both strategies are unfair and unproductive. Give credit and blame where they are properly due. I've yet to meet a person who didn't have some bias. Know yours and work hard to keep it out of evaluations.