Why does a line with a non-zero V or a cylindrical surface with a non-zero Z generate a non-cylindrical helix?

I am generating a spring as a pipe shell using a circular profile and a cylindrical helix. When I specify a U,V line with a U=0.0,V=0.0 starting point, or a cylindrical surface with a X=0.0,Y=0.0,Z=0.0 origin point (Case1) I get a cylindrical helix as expected. However, when I specify a U,V line with a V>0.0 starting point (Case2), or a cylindrical surface with a Z>0.0 origin point (Case3) I get a pear-shaped/hour-glass shaped helix which is unexpected. Why does a line with a non-zero V or a cylindrical surface with a non-zero Z generate a non-cylindrical helix? In the code below please uncomment/comment the lines with Case1, Case2 or Case3. Thanks for your response.

#include <BRepBuilderAPI_MakeEdge.hxx>
#include <BRepBuilderAPI_MakeFace.hxx>
#include <BRepBuilderAPI_MakeWire.hxx>
#include <BRepLib.hxx>
#include <BRepMesh_IncrementalMesh.hxx>
#include <BRepOffsetAPI_MakePipeShell.hxx>
#include <BRepTools.hxx>
#include <GCE2d_MakeLine.hxx>
#include <Geom2d_Line.hxx>
#include <Geom_CylindricalSurface.hxx>
#include <Standard_Real.hxx>
#include <StlAPI_Writer.hxx>
#include <TopoDS_Edge.hxx>
#include <TopoDS_Face.hxx>
#include <TopoDS_Wire.hxx>
#include <gp_Ax2.hxx>
#include <gp_Circ.hxx>
#include <iostream>

using namespace std;

int main(int argc, const char * argv[]) {

    Standard_Real profileRadius = 0.05275;
    Standard_Real helixRadius = 0.49725;
    Standard_Real centerHelixCoils = 10.0;
    Standard_Real centerHelixPitch = 0.32289;
    Standard_Real centerHelixHypotenuse = sqrt((2.0 * M_PI * 2.0 * M_PI) + (centerHelixPitch * centerHelixPitch));

    // Create Profile Face
    gp_Ax2 anAxis;
    anAxis.SetDirection(gp_Dir(0.0, -1.0, 0.0));
    anAxis.SetLocation(gp_Pnt(helixRadius, 0.0, 0.0));
    gp_Circ profileCircle(anAxis, profileRadius);
    TopoDS_Edge profileEdge = BRepBuilderAPI_MakeEdge(profileCircle).Edge();
    TopoDS_Wire profileWire = BRepBuilderAPI_MakeWire(profileEdge).Wire();
    TopoDS_Face profileFace = BRepBuilderAPI_MakeFace(profileWire).Face();

    // Create Center Helix
//    Standard_Real v = 0.0, z = 0.0; // Case1: Works
//    Standard_Real v = 0.1055, z = 0.0; // Case2: Fails
    Standard_Real v = 0.0, z = 0.1055; // Case3: Fails
    Handle(Geom2d_Line) centerHelixSegment = GCE2d_MakeLine(gp_Pnt2d(0.0, v), gp_Dir2d(2. * M_PI, centerHelixPitch));
    gp_Ax2 centerHelixOrigin(gp_Pnt(0.0, 0.0, z), gp_Dir(0.0, 0.0, 1.0));
    Handle(Geom_CylindricalSurface) centerHelixCylinder = new Geom_CylindricalSurface(centerHelixOrigin, helixRadius);
    TopoDS_Edge centerHelixEdge = BRepBuilderAPI_MakeEdge(centerHelixSegment, centerHelixCylinder, 0.0, centerHelixCoils * centerHelixHypotenuse).Edge();
    BRepLib::BuildCurve3d(centerHelixEdge);

    // Create Center Helix Wire and Pipe
    TopoDS_Wire helixWire = BRepBuilderAPI_MakeWire(centerHelixEdge).Wire();
    BRepOffsetAPI_MakePipeShell helixPipe(helixWire);
    helixPipe.Add(profileWire, Standard_False, Standard_True);
    helixPipe.Build();
    Standard_Boolean flag = helixPipe.MakeSolid();
    std::cout << "flag=" << (flag==true ? "success" : "fail") << std::endl;

    // Mesh Helix Pipe
    BRepMesh_IncrementalMesh mesh1(helixPipe, 0.05, Standard_False, 0.5, Standard_False );
    Standard_Integer status;
    status = mesh1.GetStatusFlags();
    std::cout << "status=" << status << std::endl;

    // Generate Helix Pipe BREP File
    Standard_Boolean brep_result;
    brep_result = BRepTools::Write(helixPipe, "helixPipe.brep", Standard_False, Standard_False, TopTools_FormatVersion_VERSION_1);
    std::cout << "brep_result=" << (brep_result==true ? "success" : "fail") << std::endl;

    return 0;
}

Regards, Brian D. Watt