Why cv2.GaussianBlur modify max value?

Why cv2.GaussianBlur modify max value in this case?

Here is example code:

import numpy as np
import cv2

mask = np.zeros((256, 256, 1), np.uint8)
mask[128:, :] = 255

np.max(mask)
255

mask = cv2.GaussianBlur(mask, ksize=(15, 15), sigmaX=0, sigmaY=0)

np.max(mask)
253

Update:

Seems it's related to np.uint8 type, because in np.float32 it's:

np.max(mask)
255.0
np.max(mask)
255.00002

from Why cv2.GaussianBlur modify max value?