Just to add.
One of the reasons that proponents of PPP use is that exchange rates are inherently volatile. But ask yourself this: if India's economy "shrank" by a stunning 40% using the 2005 ICP standards, how is that stable? And who says that if they increase their GDP by 40% this time, that the new standard is accurate?
Or take Vietnam whose economy "shrank" by 40%.
Exchange rates are problematic, see the so-called 'Penn effect', but this is why the Atlas method is better(but still very imperfect) as it measures GNI(Gross National Income) per capita with a smoothed 3 year exchange rate.
All of which goes to show just how unstable the economic business really is. If a country's GDP can increase by 40% or 20% just by a flip of a switch - and we still are not sure if that is even accurate - then it tells you how far we still have to go.