MFC中客户区，窗口像素的大小不统一？

//设置窗口大小
BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs)
{
if (!CFrameWnd::PreCreateWindow(cs))
return FALSE;
// TODO: 在此处通过修改
// CREATESTRUCT cs 来修改窗口类或样式

//更改窗口外观
CSize size;

//获取屏幕像素大小
size.cx = ::GetSystemMetrics(SM_CXSCREEN);
size.cy = ::GetSystemMetrics(SM_CYSCREEN);

//窗口风格
cs.style = WS_POPUPWINDOW;
//去掉菜单
cs.hMenu = NULL;
//窗口标题名称
cs.lpszName = _T("Button");

//自定义窗口大小, 并窗口居中显示
cs.x = (size.cx - 1200) / 2;
cs.y = (size.cy - 1000) / 2;
cs.cx = 1200;
cs.cy = 1000;

return TRUE;

}

//客户区大小
m_sClient.SetSize(1200, 1000);
pDC->BitBlt(0, 0, m_sClient.cx, m_sClient.cy, &m_bufferDC, 0, 0, SRCCOPY);把图片贴进去