If the corruption extends to the DAs and the police, the arrest numbers are worthless.
Yup.
And why, even if every corrupt official were caught, would working out corruption per capita even a sensible thing to do? An absolute monarchy in which the only government official with any power -- the king -- habitually takes bribes while ruling over a million people is arguably much more corrupt than a tiny city-state republic with 100,000 inhabitants, and 1000 officers of state with actual power 30 of whom can be suborned with bribes, but the per capita corruption rate in the first is .1 per 100,000, while in the latter is 30 per 100,000.