Why is Niantic giving Pokemon a 9% nerf even though they don't need it?

I honestly don't think Niantic gives it much thought beyond "oh, it has over 4k max CP? It gets the 9% nerf." What's really sad, imo, is when they also saddle some of these Mons with really bad moves.

Slaking is probably gonna get Pound for community day
Ho-Oh stats is still good he just needs better moves

Ghost types have been pretty much been a glass cannon. If giratina had both high attack and defense then there will be no point in using any other ghost types.

And what is it about the magic number (see:arbitrary) of 9% that can balance every single individual mon that is deemed to require a nerf?

The worst part about it is that since it's not dynamic, Pokemon that barely qualify for the nerf become much weaker than Pokemon that don't quite qualify for it.

There are many great Pokemon that have max CP in the high 3000s (or even mid 3000s) that end up with better stats than Pokemon with CP a touch over 4000.