Very interesting set of data... It appears as though the world is getting smarter...
I'd guess it has more to do with the spread of information via the internet and a big increase in the availability of study materials and courses.
I know my dad took the GMAT back in the day (probably around the same time) and he claims 90% of people would take the test cold, basically just wing it. Don't know if that's accurate but I do believe that today a much larger portion of people use study materials and prep courses. You have to now to get a half-decent score.
Anyway, its interesting to see the data.