Come on, Richard. The differences in the 90's between countries wasn't that significant. There were just as high scores in South Africa in the 90's as there were in Australia as there were in the WI. Subcontinental pitches haven't changed for many years now and England's decks have improved too but largely due to the time of year Tests are played these days.
None flatter, but that's not the point - it's only the last 6 years that surfaces Worldwide have become universally flat. Before then there was still a huge amount of variation country-to-country, even if not as much as in the days of uncovered surfaces in some countries (and at one point in all) and matting wickets in some countries.