为什么我的角色移动感觉不对？

// 假设角色移动的代码如下
function moveCharacter(direction, speed) {
    switch (direction) {
        case 'up':
            character.y -= speed;
            break;
        case 'down':
            character.y += speed;
            break;
        case 'left':
            character.x -= speed;
            break;
        case 'right':
            character.x += speed;
            break;
    }
}

// 假设使用Box2D物理引擎
var bodyDef = new Box2D.b2BodyDef();
bodyDef.type = Box2D.b2_dynamicBody;
bodyDef.position.Set(character.x, character.y);
var body = world.CreateBody(bodyDef);

var fixDef = new Box2D.b2FixtureDef();
fixDef.shape = new Box2D.b2PolygonShape();
fixDef.shape.SetAsBox(character.width / 2, character.height / 2);
body.CreateFixture(fixDef);

// 假设处理键盘输入的代码如下
document.addEventListener('keydown', function(event) {
    switch (event.key) {
        case 'ArrowUp':
            moveCharacter('up', speed);
            break;
        case 'ArrowDown':
            moveCharacter('down', speed);
            break;
        case 'ArrowLeft':
            moveCharacter('left', speed);
            break;
        case 'ArrowRight':
            moveCharacter('right', speed);
            break;
    }
});