I kindly disagree.
It could just be because Mercury is the oldest member.
The Gorosei are subordinates of Imu, who is more than like represents Mother Earth, and not Nika (Sun), so it makes more sense for their order to be according to their closeness to the Earth.
I think this pic (bottom left) is a clear indication to their hierarchy.
We have:
1- Imu (obviously)
2- Venus/Mars (Venus imo)
3- Mars/Venus (Mars imo)
4- Jupiter/Mercury (Jupiter imo)
5- Jutiper/Mercury (Mercury imo)
6- Saturn