Unexpected deformation when exported the result geometry

When I finished the fluid solid coupling, I tried to remesh the deformed configuration. Here comes the question, when I export the mesh of the initial time, it is supposed to be the exactly same with the original geommetry,however, it had a very slight change which baffled me for a long time.  I have tried variety of ways ending up with failure.
  I cannot figure it out and need your help sincerely!