Actually the Seat matrix are controlled by certain factors like trend, ease of availability so that they can choose higher options.
When the Counselling progresses people will be going to other branches due to up-gradations and there will be Seats left.so thats why there is always a change in the Seats remaining.
FACT: every year there will be Seats left after Real admissions and CUSAT attempts to fill the same with Spot admissions. Even though some years the seats still remain unfilled.