Actaea

Is Freedom From Corruption just Corruption with a negative sign?

It seems so, yes.

It's completely unclear why such a statistical characteristic is needed.

So that achievements are not only received by the most corrupt regimes, but also by the most honest ones.

And also, tell me what Averageness means.

It means - where the most average citizens live. There's nothing special to say about them. They have two children, a boy and a girl, the father is 45 years old and brings in most of the family income, the mother is 38 years old, but she lies to everyone that she is 35, she works in an office in a minor position, etc.