I am using Slick2d to create a top-down game, however an rectangle than I am drawing to the display is moving faster to the left than to the right, and faster up than down.

I am updating frames like so:

    public void update(GameContainer gameContainer, int delta) throws SlickException { 
        player = new Rectangle(playerX, playerY, 10, 20);

        if (gameContainer.getInput().isKeyDown(Input.KEY_LEFT)) {
            x -= delta * 0.1f;
        }
        if (gameContainer.getInput().isKeyDown(Input.KEY_RIGHT)) {
            x += delta * 0.1f;
        }
        if (gameContainer.getInput().isKeyDown(Input.KEY_UP)) {
            y -= delta * 0.1f;
        }
        if (gameContainer.getInput().isKeyDown(Input.KEY_DOWN)) {
            y += delta * 0.1f;
        }
    }

I then simply draw the Rectangle to the display. It is noteworthy that the Rectangle class is a 
>org.newdawn.slick.geom.Rectangle

**Why are the speeds for the movement different, and how can I fix this?**

***EDIT:*** I have fixed the problem by slowing the movement to the left and up, but this does not explain why the problem occurs