The Tompkins table is calculated based on all years. So:
1. a college will do better if it admits students who will get a first every year or who will get a first in the first 2 years then graduate with 2.1, rather than admitting students who will get a first in the final year but have to travel further to get there, with lower marks on the way.
2. As far as I know, a college will do better if it admits more students in subjects where more firsts are awarded.
...and as has been said, the applicants colleges get can look quite different from one college to the nexT