surface normal

Dear all,

I'm using abaqus to build a spherical model, which applies C3D8 linear brick element.  I output the connectivities of one element on the surface and the corresponding coordinates. 

For a linear brick element, the four nodes A,B,C and D on the same surface should lie on the same plane.  However, I got two different normals which are from A,B,C as well as A,B,D.

Is it reasonable or do I misunderstand something?

Best