Why do smaller terrains have higher triangle count?

I must be overlooking something...

I created this test of unity terrains, no details, no splats:

    grid   res   fps    ram    tris
 1: 2x2    1024  1030   1126   680k
 2: 4x4    512   385    1133   2.2m
 3: 8x8    256   84     1223   4.6m

those are the amount of terrains in scene, the terrain resolution, the fps on a stationary camera, the memory use of the editor in play mode, and the visible triangle count.

Okay but...

 1024x1024x2x2 = 4,194,304
 512x512x4x4   = 4,194,304
 256x256x8x8   = 4,194,304

why is the vertex count doubling in play when they should all be the same?... we're talking about heightmap resolution of course... I don't get it? I must be missing something stupid here...