新手，do while.....loop小问

Dim i, sum As Integer
        i = 1
        sum = 0
        Do While i <= 100
            sum = sum + i
            i = i + 1
        Loop
这个sum的值是5050
一下只是变了一下顺序
Dim i, sum As Integer
        i = 1
        sum = 0
        Do While i <= 100
            i = i + 1   
            sum = sum + i
        Loop
只是变了一下顺序，sum的值就变成了5150，多了100，这是为什么啊？！！！！