Appreciation for members

I don't understand why a lead always prefer to appreciate a single person in a team. In my team I observed the lead is appreciating a few favorite people, due to which the over all team gets demotivated. We should be transparent and very much careful while typing an appreciation note for a member as without a very relevant reason we should not appreciate people. As in a team there are good, average and poor people use to be there but we can't ignore each and every performance as end of they team delivers the solution. We can't loose these people as we know we invest a lot to get them on track. If that is the case then we need to retain them through good appreciations or rating. Obviously rating is very important but as we know a institute can't give good rating to all and always. In that case these appreciations/gems known as indirect rating, are the only solutions. We need to appreciate or award those people for whom we can't sure about good rating as we middle manager better understand what is going to happen to their ratings. As we want to retain these average people, we should arrange some appreciations/awards for them. I am not saying we should not appreciate good people, we are not that stupid but think about how we can keep the good team spirit and retain those valuable people through these kind of quantitative appreciations and indirect ratings. I hope people will understand the organization issue instead of personal and personal goal.