GDI+和GDI坐标系统改变导致长度不一致有关问题

GDI+和GDI坐标系统改变导致长度不一致问题
使用GDI的SetMapMode(MM_LOMETRIC)和GDI+的SetPageUnit(UnitMillimeter)设置单位长度后都画20mm线，但出来的长度竟然不相同，郁闷。
各位帮忙解释下为什么。

------解决方案--------------------
void CDrawView::OnDraw(CDC* pDC)
{
CDrawDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
// TODO: add draw code for native data here
CRect rect;

// map
GetClientRect(rect);
pDC-> SetMapMode(MM_ISOTROPIC);
pDC-> SetWindowExt(100000, -100000);
pDC-> SetViewportExt(rect.Width(), rect.Height());
pDC-> SetWindowOrg(0, 0);
pDC-> SetViewportOrg(rect.left, rect.top + rect.Height());

// GDI draw polyline
POINT pt[5];
pt[0].x = 10000;
pt[0].y = 20000;
pt[1].x = 80000;
pt[1].y = 20000;
pt[2].x = 80000;
pt[2].y = 60000;
pt[3].x = 10000;
pt[3].y = 60000;
pt[4].x = 10000;
pt[4].y = 20000;
pDC-> Polyline(pt, 5);

// GDI+ draw polyline
Point pos[5];
pos[0].X = 10000;
pos[0].Y = 20000;
pos[1].X = 80000;
pos[1].Y = 20000;
pos[2].X = 80000;
pos[2].Y = 60000;
pos[3].X = 10000;
pos[3].Y = 60000;
pos[4].X = 10000;
pos[4].Y = 20000;

Graphics graphics(pDC-> m_hDC);
Pen blackPen(Color(255, 255, 0, 0), 1);
graphics.DrawLines(&blackPen, pos, 5);
}

两个多边形坐标点一样,结果是两个多边形不重合? 在文本映射模式下没有问题