This is where the use of the word "seat" is imprecise. LH has no less than 9 reservations to sell in PE on the flight. There may be somewhere from 0 to 9 seats presently available. LH uses sophisticated RM/IM software which might, by way of example, predict that for your flight, typically 9 passengers will cancel, misconnect, die or otherwise no show for you flight and thus LH may overbook by 9. Perhaps there are sufficient seats in F/J to OPUP PE passengers if the predictive algorithm is incorrect or perhaps some would need to be downgraded or rebooked. There is simply no way to tell and the algorithms are sensitive proprietary information.