Timer Interval Is No Accurate! Why???

[qazwsx123]

Hi Every One.
As I know 1 Sec. is 1000 milliseconds so in the code blow the final number in label must be 1000:

Dim a As Integer

Private Sub Form_Load()
a = 0
Timer1.Interval = 1
Timer2.Interval = 1000
End Sub

Private Sub Timer1_Timer()
a = a + 1
End Sub

Private Sub Timer2_Timer()
Timer1.Interval = 0
Label1.Caption = a
End Sub

But the label shows 129 !!!!!!
Why???
Can any one tell me?