诸位大侠，在下不才，请告诉我程序的运行流程好吗？

寒假复习VB准备考二级，看书的时候做课后习题遇到困难了。
执行下面程序，当单击窗体时，输出的结果是什么？
Private Sub Form_click()
    Dim I As Integer, k As Integer
    Dim a1 As Integer, a2 As Integer, a3 As Integer
    a1 = 1: a2 = 10: a3 = 2
    For I = a1 To a2 Step a3
        k = k + 1
        I = I + 2
        a1 = a1 + 1
        a2 = a2 - 5
        a3 = a3 + 2
    Next I
    Print k, I
    Print a1, a2, a3
End Sub
我算了几遍，和运行结果都不一致，每个数值都算的不对。我不明白I的取值到底是怎么样的顺序和流程，请各位大神给指点一下，多谢多谢！
PS：运行结果是       3                       13
                     4                       -5                    8