At my company it wasn't a tenure thing, although I think that played an informal role as to why I was picked vs someone else. When a manager picks someone for the highest rating some departments require the manager to also pick someone as an FDN to "balance it out".

In this case I know the guy that got the high rating. As weird as it is to say, and even when the wound was fresh, I was/am happy for him and he deserved it. He's a nice guy and he had nothing to do with the rating I recieved. He's now a manager several levels up.