Here is the pattern I've seen, which doesn't really line up with the OP's premise.
Radeon 5 series released, AMD tops.
time passes AMD winner
Nvidia 480 released, nvidia retakes top.
times passes NVIDIA WINNER
Radeon 6 series released, AMD tops.
time passes AMD winner
Nvidia 580 released, nvidia retakes top.
times passes NVIDIA WINNER
Radeon 7 series released, AMD tops.
time passes AMD winner
Nvidia 680 released, nvidia retakes top.
times passes NVIDIA WINNER
I don't see domination at all, seems to me that every new release changes the current winner. Since this thread was posted before the release of AMD's 8 series nvidia happens to be in the lead, but that will change in a few months.