If you don't create a root window, Tkinter will create one for you when you try to create any other widget. Thus, in your __init__, because you haven't yet created a root window when you initialize the frame, Tkinter will create one for you. Then, you call make_widgets which creates a second root window. That is why you are seeing two windows.
A well-written Tkinter program should always explicitly create a root window before creating any other widgets.
When you modify your code to explicitly create the root window, you'll end up with one window with the expected title.
Example:
from tkinter import Tk, Button, Frame, Entry, END
class ABC(Frame):
def __init__(self,parent=None):
Frame.__init__(self,parent)
self.parent = parent
self.pack()
self.make_widgets()
def make_widgets(self):
# don't assume that self.parent is a root window.
# instead, call `winfo_toplevel to get the root window
self.winfo_toplevel().title("Simple Prog")
# this adds something to the frame, otherwise the default
# size of the window will be very small
label = Entry(self)
label.pack(side="top", fill="x")
root = Tk()
abc = ABC(root)
root.mainloop()
Also note the use of self.make_widgets() rather than ABC.make_widgets(self). While both end up doing the same thing, the former is the proper way to call the function.