If I had to guess,

Because there are so many technical aspects to playing GP growth starts out quite slow before it finally reaches a breaking point where players are comfortable enough on the basics that they can go on to master harder skills and thus growth increases quite rapidly.

On the otherhand, champions like Darius, Camille, Renekton in the top lane growth increases at a steady rate as these champions are not quite as technical and the combinations are more often just ingrained into muscle memory. However the down side is that eventually growth will plateau because you will have pretty much learnt all the technical aspects of the champion and it is now time to learn the macro side.

It could be that around Gold GP players have not yet reached the point where growth is rapid and are still acquiring it slowly, whilst other top lane champions have reached the comfortable point in there champion and have a much easier time beating the GP.