It's a perceptions index, carried out by a Western organization, polling almost exclusively other Western organizations, that deemed Western nations are just awesome. I am not suggesting that e.g. China is not corrupt (nor that Denmark is not awesome), but I am suggesting that it's as objective as poll of Eastern organizations on the same question - which is to say, not at all.

The unfortunate thing is that corruption is in many cases all but impossible to prove or quantify. And in some cases there is a mixture of corruption and genuine interest. As one example a war that furthers national interests, but also ensures rich rewards for companies of which you will receive returns from (indirect or otherwise) ends up in a balancing act of trying to measure immeasurables - perceived self interest vs perceived national interested.