Why a 2D and 3D model using a coupled thermal displacement element in ABAQUS give different results? Can any one help me?

I am modeling a rectangular plate placed on a hot plate using a coupled temperature displacement type of analysis in abaqus. I have modeled it in 2D as well as 3D. I have defined a contact between hot plate and rectangular specimen. But my 2D and 3D results are different. Can any one help me in clearing my doubt. Thanks in advance.