physics body not inline with shape?

for some reason when i apply a physics body around the skshapenode it places the physics body more to the left and downwards of the actual shape even though the parameters of the physics body comes from the shape. does anyone know why? thanks.

-(SKShapeNode*) createGround1
{
    //create a rectangle
    ground1 = [SKShapeNode shapeNodeWithRect:CGRectMake(0, 0, 300, 19)];
    ground1.position = CGPointMake(800, 500);

    //applies physics to rectangle
    ground1.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize: CGSizeMake(ground1.frame.size.width, ground1.frame.size.height)];
    ground1.physicsBody.allowsRotation = false;
    ground1.physicsBody.dynamic = false;
    ground1.physicsBody.restitution = 0.0;

    return ground1;
}