[Python] Why is cv2.imencode('jpg', ...) seemingly changing the color of an image?

I create and view a random image. Then, this image is encoded as a jpg file with opencv. However, after decoding this image the colors have been changed slightly. This behavior is not seen using png to encode. Can anyone explain why this occurs? Is it a negative result of the jpeg compression? Am I doing something wrong? Code sample below to recreate this.

import cv2
import numpy as np

random_image = np.random.randint(255, size=(4,4,3), dtype=np.uint8)
cv2.imshow('Image', random_image)
cv2.waitKey()

_, img_encoded = cv2.imencode('.jpg', random_image)
img_string = img_encoded.tostring()
npimg = np.fromstring(img_string, dtype=np.uint8)
img = cv2.imdecode(npimg, 1)
cv2.imshow('Image', img)
cv2.waitKey()

# Does not happen with png
_, img_encoded = cv2.imencode('.png', random_image)
img_string = img_encoded.tostring()
npimg = np.fromstring(img_string, dtype=np.uint8)
img = cv2.imdecode(npimg, 1)
cv2.imshow('Image', img)
cv2.waitKey()



Edited to add some 4x4 images.

Original:



JPG



PNG



Edited again with 512x512 images

Original 512x512

JPG 512x512

PNG 512x512

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