I have seen posts by some of these people saying they have not been awarded any bonus because the scheme...while it is in this 'forced distribution' mode, it means that it is not possible for all the group to be found to be performing adequately or meeting all standards...the bottom 10% has to be under the bar.
I think, after a period it is supposed to change to a mode where the bottom 10% are also deemed to be possibly good at their jobs too...
It reminds me of the very old A level and O level system....followed by the shift to the current one.
Tragic for any department that is made up of really good people, but maybe department heads have the ability to plead exceptional group performance?