I think it's directx that batches the triangles like that. Basically keeps your triangles at 2000 at the most for each surface. I did alot of tests on that, on different computers and they seemed to yield the same result. Small FPS increases, everytime i added more triangles to a surface.
When i hit > 2000, the FPS was slightly greater than the FPS that came before hand.
When i hit > 2000, the FPS was slightly greater than the FPS that came before hand.