Why is so many outside Font down graded?

The history shows that almost 50 % of all the hardest boulders in most hard core areas but Fontainebleau are actually down graded. Why is it that often the FAs think hard boulders, outside Font, are harder compared to the repeaters, i.e. an 8A+ FA often becomes 8A? Which are the larger boulder areas with the lowest percentage down gradings? The Fontainebleau boulders which are very seldom down graded are often quite vertical with slooper or mantle cruxes which is like the opposite with the boulders that are down graded in like 50 %. Is the best explanation for the low proportion of down grades that the Font style is more difficult to train in the gym?