VB.net里面怎么让图形不超出窗体边界

用timer控制picturebox在移动，图形碰到边界后返回，代码是这样的

Public Class Form2
    Dim xi As Integer, yi As Integer
    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        Dim x As Single, y As Single
        x = PictureBox1.Location.X
        y = PictureBox1.Location.Y
        If x > Me.Width - PictureBox1.Width - 16 Or x < 0 Then
            xi = -xi
        End If
        If y > Me.Height - PictureBox1.Height - Me.Icon.Height - 6 Or y < 0 Then
            yi = -yi
        End If
        x = x + xi
        y = y + yi
        PictureBox1.Location = New Point(x, y)
    End Sub

    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Timer1.Enabled = True
        Timer1.Interval = 30
        xi = 1
        yi = 1
    End Sub
End Class

这样恰能完美运行，我比较奇怪的必须x和y坐标的边界位置必须分别减去16和6，不然运行起来就会超出窗体边界才回来

我想要知道这究竟是为什么，窗体长这样

要算form的边框和标题栏