Immensely wealthy multinational corporations have a lot of money to spend on lawyers and accountants, so while these companies are probably not evading tax the avoidance schemes they come up with are sometimes not legal requiring them to repay.

Don't forget that tax is part of law and so there is what's written and how that's interpreted by the courts. Until you've had case law there are areas of uncertainty.

It's not a hard number but perceptions of fairness are certainly important in how democratic countries formulate tax law.