Why does the Tkinter canvas request 4 extra pixels for the width and height?

>>> import Tkinter
>>> c = Tkinter.Canvas(width=100, height=100)
>>> c.winfo_reqwidth()
104
>>> c.winfo_reqheight()
104

The results are the same if I set borderwidth to zero. I can't find the setting or property that explains or controls these 4 extra pixels.