Newbie question about Tkinter

[Andr? Roberge]

The following program (Python 2.3, under Windows XP)
===
import Tkinter as Tk
A = Tk.Tk()
A.title("1")
A.mainloop()
B = Tk.Tk()
B.title("2")
B.mainloop()
===
opens window A and waits for it to be closed before opening window B.
However, the following opens both windows "simultaneously".  I tought
it would give the same result as the one above... I am confused.
====
import Tkinter as Tk
A = Tk.Tk()
A.title("1")
B = Tk.Tk()
B.title("2")
A.mainloop()
B.mainloop()
====
Anyone can explain or give a pointer to the answer.
André