People here do not get it, it is a corruption perception index not a corruption index.
https://en.wikipedia.org/wiki/Corruption_Perceptions_Index
Corruption perception index ranks countries by their perceived levels of public sector corruption, as determined by expert assessments and opinion surveys.
There is certainly a correlation between corruption and corruption perception, however that also means that countries that are good at hiding it, will have a betterr corruption perception index.
Perception = not truly objective
It is not because you don't see it that it does not exist.
Of course people that live in lands where cops are paid 50$ a month a witness tham taking bribes in broad daylight, will have a "bad" corruption perception.
Ironically, the NGO that publishes this study , Germany based Transparency International, is itself pretty corrupt..