The problem with that index is that each culture reacts differently to corruption. In some cultures, if a public servant buys a coffee using the company card, that's a scandal, and some of those cultures have a reputation for being corrupt.
In Germany, everyone downplays corruption for some reason. But I see it everywhere, especially in everything that had to do with public funds. But it's never called corruption, so corruption does not exist because it is never acknowledged.
https://www.transparency.org/en/news/cpi-2023-highlights-ins...
Measuring corruption versus measuring perception of corruption. The former requires evidence of corruption.