There are rounds of counselling. For example you scored 500 in an entrance and there are 5 colleges ranked. According to your score, you will get college no. 3 in the first counselling and others with a higher score than you will get no. 1 and no. 2. Now one of the candidate for some reason has left the college no. 2 that he got in counselling round 1. That seat is vacant now. Now, in the second round of counselling, your seat that you got will get upgraded to college no. 2 from 3. 

That's how a counselling process occurs for almost all the entrances. Basically, upgradation of seats in various colleges occur from the one that you get in round 1.