As has been covered by previous posters, performance related pay was forced upon the MoD CS by the Treasury, the hidden agenda being, as already stated, to cut the pension bill. It was universally hated and seen as devisive. Because of this it's been tinkered with every year since its inception. Currently, all get a standard bonus unless on restoring efficiency for whatever reason. The number of higher awards have been trimmed so you have to be doing something special to receive one. As you have to write your own evidence and your line manager adds their bit, sometimes it can hinge on how good both of you are with the written word!! That in itself is why it can be so unfair to some.
IMHO the majority of MoD CSs do a good job for little reward.