Every year available seats to be filled through CSS are announced and allocated to provinces as per fixed quota, in case qualified candidates are less in number than available seats, obviously the seats will remain unfilled as its due share of particular region, how can these be filled by allocating the candidates from other regions/provinces? Hence unfilled vacancies are carried forward to the next year. As the population of Punjab is more hence more candidates apply and and get good merit number hence they fill not only merit seats but also their due share and eventually are no seats are left vacant, same is the case with KPK.

__________________
Your attitude, not your aptitude, will determine your altitude...Zig Ziglar