I think it is due to the Gold 99 being slightly heavier, but also slightly faster feeling with a lower swingweight and more head light feel that has me getting more pace on my shots. But the difference is only slight.
The 100 is a much more modern feeling racquet. I would say it is a more comfortable and feel oriented Pure Drive, but lacks the pop, crispness and stability of the Pure Drive.
The Gold 99 is a much more traditional feeling racquet in its response. It is also very comfortable, feels dampened yet does not leave the player feeling disconnected. It has more pop than the X-Red it replaced.
Hope that helps,