Why is the solution not symmetric?

Hp

Hello,

I conduct the simulation of heat transfer inside a rectangular tank. The BC is symmetric, so are the geometry and the mesh. I used UDF for the source term and the source term is set for each cell, which I do not think it will affect the symmetry of the problem. So I expect the pattern of the flow/thermal solution is also symmetric or have mirror symmetry. However, the solution is not symmetric. Can anyone tell me the reasons?

Thanks!

HP