Python-OpenCV get(CAP_PROP_POS_AVI_RATIO) returning a fixed number in while loop?

Question

I have tried to analyze video files (avi files) using OpenCV Python. In particular, I tried to see the percentage of frames processed as specified in CAP_PROP_POS_AVI_RATIO, which should return "Relative position of the video file: 0 - start of the film, 1 - end of the film."

However, when I try to put CAP_PROP_POS_AVI_RATIO in the VideoCapture code as followed, the AVI_RATIO only return a fixed number of 0.05. When I change the avi file to others, the AVI_RATIO is different for different avi files, like 0.1 or 0.0285714285714 etc.

import numpy as np
import cv2

playvideo = cv2.VideoCapture('output.avi')
while(playvideo.isOpened()):
    ret, frame = playvideo.read()
    cv2.imshow('window', frame)
    if cv2.waitKey(100) & 0xFF == ord('q'):
        break
    video_ratio = playvideo.get(cv2.CAP_PROP_POS_AVI_RATIO)
    print(video_ratio)
    video_frame = playvideo.get(cv2.CAP_PROP_POS_FRAMES)
    print(video_frame)
playvideo.release()
cv2.destroyAllWindows()

The CAP_PROP_POS_FRAMES on the other hand works fine, ie. returning the frame per iteration.

This is very strange. How come the cv2.CAP_PROP_POS_AVI_RATIO is not giving the ratio, but a fixed number for a video file?