Basically the funda is to complete 5000 seats but unfortunately due to wrong choice filling or we can say bad luck students don't get college allotted in the allotment list. Maybe they fill only 5-10 colleges or the colleges they fill are taken by the other students. So now the remaining seats are filled and released under other rounds. Usually they try to cover in 2 rounds but sometimes it may go to 3-4 rounds.
Also if students get college in first round many students use float option and do not freeze the seat so this is also the reason due to which number of rounds are affected