Why is inner padding only applied on the right?

The current answers provide a workaround by using the padding keyword argument. An explanation WHY using ipadx fails is still needed.

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MCVE

import Tkinter as tk
import ttk

root = tk.Tk()
root.minsize(200, 100)
inner_frame = ttk.LabelFrame(root, text='inner_frame')
inner_frame.grid(row=0, column=0)

button = ttk.Button(inner_frame, text='this is a button')
button.grid(row=0, column=0)

# this does not work as expected
inner_frame.grid_configure(ipadx=20)

root.mainloop()

Output

Question

Why is the inner padding for inner_frame only applied on the right? How do I apply it on both sides?

from Why is inner padding only applied on the right?