I understand, however it was a disclaimer not a "trust me".
One of the main reason such metrics are not as reflective of the reality is that for developing countries, the taxes are not reported accurately. A lot of wealth is officially unreported - and it is a massive amount. Such things are not captured by official statistics, by virtue of being unreported income, which leads to an incomplete or distorted picture of reality.
In absolute terms though just look at pictures of e.g. rural Russia va Moscow.