unexpected shell conduction
 

Hi,
My project is a 3D model with two zones, solid zone and fluid zone respectively. I am trying to study the conduction across the contact surface. But the result seems like shell conduction. I didn't tick the shell conduction in boundary condition. And I have realized that when I import meshes into Fluent, it create shell conduction zone automatically.
I have attatched the temperature profile. The dark blue part is the fluid part, while the bottom and top is solid part which is connected.
What's wrong with it?
Thanks.