Because the Tiger Compiler is a project with stages, the computation of the marks depends on the stages too. To spell it out explicitly:
A stage is penalized by bad results on tests performed for previous stages.
It means, for instance, that a TC-3 compiler will be exercised on TC-1, TC-2, and TC-3. If there are still errors on TC-1 and TC-2 tests, they will pessimize the result of TC-3 tests. The older the errors are, the more expensive they are.