how to get correct shape location

hello , I met a problem when trying to get the Z locaion of a plane by selection. I am using OCCT 7.7 on ubuntu 22.04. I made a window to show the step file, and selected a face on the body. I want to get the Z location of the face but got two different result which really makes me confused. 

// this code is a part of function setShapes in class Algo    

    occtviewer->Context()->InitSelected();  // occtviewer->Context() gets the context, occtviewer is an instance of class OcctQtViewer
    Handle(StdSelect_BRepOwner) owner = Handle(StdSelect_BRepOwner)::DownCast (occtviewer->Context()->SelectedOwner());
    TopoDS_Shape shape = owner->Shape();

    up_plane = TopoDS::Face(shape);  // up_plane is a member of class Algo

    Handle(Geom_Surface) surface = BRep_Tool::Surface(TopoDS::Face(up_plane));
    Handle(Geom_Plane) plane = Handle(Geom_Plane)::DownCast(surface);
    gp_Pnt origin = plane->Location();
    std::cout << "zup = " << origin.Z() << std::endl;

// output: zup = -243.36
// Here the output is correct.

// this code is a part of function showRes in class Algo

    Handle(Geom_Surface) surface = BRep_Tool::Surface(TopoDS::Face(up_plane));
    Handle(Geom_Plane) plane = Handle(Geom_Plane)::DownCast(surface);
    gp_Pnt origin = plane->Location();
    double up_z =  origin.Z();
    std::cout << "up_z= " << up_z << std::endl;

// output: up_z= -314.455
// Here the output is wrong.

Function setShapes is executed before function showRes.

I really want to know what's wrong with my code  and solve this problem.