Непонятное поведение BitBlt на разных виндах 7

Private Sub Form_Load()
 
    Hwndr = Me.hWnd
    HDCR = GetDC(Hwndr)
    
    GetWindowRect Hwndr, rct
    GetClientRect Hwndr, InnerRct
    
    ResultWidth = Abs(InnerRct.right - InnerRct.Left)
    ResultHeight = Abs(InnerRct.bottom - InnerRct.Top)
    
    With bi24BitInfo.bmiHeader
    .biBitCount = 24
    .biCompression = BI_RGB
    .biPlanes = 1
    .biSize = Len(bi24BitInfo.bmiHeader)
    .biWidth = ResultWidth
    .biHeight = ResultHeight
    End With
    
    BackHDC = CreateCompatibleDC(HDCR)
    Back = CreateDIBSection(BackHDC, bi24BitInfo, DIB_RGB_COLORS, ByVal 0&, ByVal 0&, ByVal 0&)
    SelectObject BackHDC, Back
    
    MWND = CreateWindowEx(WS_EX_TRANSPARENT, "STATIC", "EmptyWndClass", (WS_POPUP + SS_OWNERDRAW), rct.Left, rct.Top, ResultWidth, ResultHeight, Hwndr, 0, HDCR, cs)
    mdc = GetDC(MWND)
    
    SetWindowPos MWND, Hwndr, rct.Left, rct.Top, 0, 0, (SWP_NOSIZE + SWP_NOACTIVATE + SWP_SHOWWINDOW)
 
End Sub
 
 
Private Sub Form_Unload(Cancel As Integer)
 
    ReleaseDC Hwndr, HDCR
    
    DeleteDC BackHDC
    DeleteObject Back
    
    DestroyWindow MWND
 
End Sub
 
Private Sub LabelTest_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
 
    BitBlt BackHDC, 0, 0, Abs(rct.right - rct.Left), Abs(rct.bottom - rct.Top), HDCR, 0, 0, SRCCOPY
 
    BitBlt mdc, 20, 0, Abs(rct.right - rct.Left), Abs(rct.bottom - rct.Top), BackHDC, 0, 0, SRCCOPY
    
End Sub

BitBlt mdc, 20, 0, Abs(rct.right - rct.Left), Abs(rct.bottom - rct.Top), BackHDC, 0, 0, SRCCOPY