Как вызывающая программа может влиять на результат функции WindowFromPoint

Imports System.Text ' для StringBuilder
Private Declare Function WindowFromPoint Lib "user32" (ByVal xPoint As Integer, ByVal yPoint As Integer) As Integer ' отпределение окна под курсором мыши
Private Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hwnd As IntPtr, ByVal lpClassName As String, ByVal nMaxCount As Integer) As Integer ' получение класса окна
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
                Const nChars As Integer = 256
                Dim hWnd As IntPtr = 0
                Dim Buff As String = StrDup(256, " ")
                'hWnd = WindowFromPoint(Cursor.Position.X, Cursor.Position.Y) 
                hWnd = WindowFromPoint(1500, 900) ' + -получает хэндл окна под курсором мыши 
                GetClassName(hWnd, Buff, 256)
                Debug.Print("hWnd  " & hWnd.ToString)
                Debug.Print("GetClassName  " & Buff.ToString & vbNewLine)

Structure POINTAPI
        Public X As Int32
        Public Y As Int32
End Structure
Declare Function WindowFromPoint Lib "user32" (ByVal xPoint As POINTAPI) As IntPtr