in the first round of counselling those with Air 1 to 5000 are alloted seats based on their preferences.Now some may join and some may not due to variety of reasons.Supposing in 1st round only 2500 seats are filled,the rest 2500 are still vacant.In the second round again the seats are alloted on basis of top rankers getting their first preferences and ranks comimg after that get the seats that are still free..Now in the second round again some may not join.So if  only 1000 join there will be a third round of counselling. In this round remaining 1500 are filled.