moving bullets up the screen using y-coordinate

[allusernametaken]

Hello, python world.
I'm studying python using the book called 'Python Crash Course'.
There's a portion of the book making a game using Pygame and I'm at the part to code bullet module for a ship to fire a bullet.
Here's a bit of the codes I'm confused about.

def __init__(self, ai_game):
    --snip--
    # Create a bullet rect at (0, 0) and then set correct position.
    self.rect = pygame.Rect(0, 0, self.settings.bullet_width,
        self.settings.bullet_height)
    self.rect.midtop = ai_game.ship.rect.midtop
    --snip--
def update(self):
    """Move the bullet up the screen."""
    # Update the decimal position of the bullet.
    self.y -= self.settings.bullet_speed

and, the last line "self.y -= self.settings.bullet_speed",
Why minus? the bullet's starting position is at the bottom and it's supposed to go up along the y-axis. what am i missing?