nVidia has tighter control of it's board makers and has created enough brand loyalty to muscle itself some nice margins. Being able to lock out a company from guaranteed sales can be quite a strong negotiating tool, just look at the fate of BFG. No telling if they also get a better deal from TSMC but it would not surprise me if they do.
That and margins on their HPC products are supposedly to thank for balancing out the lower end. You can see AMD has been wanting to get a larger piece of that action, but it requires pretty drastic changes and so they have announced Graphics Core Next as their new architecture.
Well but to get brand loyalty you need to make your current consumers happy.
Which means they are doing something right.
