[Help] Understanding pygame.display.update(rects)

I recently began using PyGame as a fun side project in Python. Yesterday, I came across some documentation about the DirtyRectangle Animation technique and tried implementing it in a render method. I noticed that even though I was only supposed to be updating the section of the screen that fell under the rectangle, the entire screen would actually update which is unexpected behavior and seems to defeat the purpose.

Here is a simplified version of my original code that confused me. I am blitting three rectangles onto the top third of the screen but never updating that portion of the display, yet those rectangles appear when using pygame.display.update(self.updatedRects) when updatedRects doesn't include the top third of the screen. You can test this yourself but running my code and clicking in the bottom 2/3rds of the screen.

import pygame
import sys

pygame.init()
screenInfo = pygame.display.Info()
clock = pygame.time.Clock()
windowSize = (1000, 1000)
screen = pygame.display.set_mode(windowSize)


class GameConstants:

    windowSize = windowSize
    board_size = 3
    white = (255, 255, 255)


class Board:

    def __init__(self):
        self.size = GameConstants.board_size
        self.location = (0, 0)
        self.rectSize = (round(GameConstants.windowSize[0] / GameConstants.board_size) + 1,
                         round(GameConstants.windowSize[1] / GameConstants.board_size) + 1)
        self.rects = []
        for row in range(3):
            y = self.rectSize[0] * row
            for col in range(3):
                x = self.rectSize[1] * col
                self.rects.append((pygame.Rect((x, y), self.rectSize), (255 // (row + 1), 255 // (col + 1), 200)))


class Game:

    def __init__(self, board):
        self.board = board
        self.piecePos = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
        self.updateRects = []

    def newPiece(self, mousePosition):
        pos = self.convertPos(mousePosition)
        if pos and self.piecePos[pos[0]][pos[1]] == 0:
            x, y = pos
            self.piecePos[x][y] = 1

            # blit on screen
            rect = self.board.rects[x + y * 3]
            screen.fill(rect[1], rect[0])
            self.updateRects.append(rect[0])

    def render(self):
        print(self.updateRects)
        pygame.display.update(self.updateRects)
        self.updateRects = []

    def convertPos(self, position):
        for index, rect in enumerate(self.board.rects):
            if rect[0].collidepoint(position):
                return (index - 3 * (index // 3), index // 3)

    def reset(self):
        self.piecePos = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
        screen.fill(GameConstants.white)
        pygame.display.flip()
        for rect in board.rects[0:3]:
            screen.fill(rect[1], rect[0])


board = Board()
game = Game(board)
game.reset()

while True:
    clock.tick(40)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()
        elif event.type == pygame.MOUSEBUTTONDOWN:
            mousePosition = pygame.mouse.get_pos()
            game.newPiece(mousePosition)
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_ESCAPE:
                sys.exit()
            elif event.key == pygame.K_r:
                game.reset()
    if game.updateRects:
        game.render()