Which surface the terminal voltage should apply to in the electrostatics analysis?

I want to calculate the electric capacity between two metal plates then I do a model in comsol as follow:

I draw two blocks with a 2µm gap between them,and the material of the two blocks are set to aluminum,and the gap is filled with gas. then the boundary conditions of "Ground" and "Terminal" should be the surfaces of blocks away from gas(surface A and D in schematic diagram) or the surfaces of blocks overlap with gas(surface B and C in schematic diagram)? I got two different results in the conditions above mentioned,and why? the two blocks are metal and should be a equipotential body in the real world and why i got a different result in the comsol?
thanks a lot!