It's not as simple as that, take this as an example (for maths, simply because I know the specific modules)
Student 1
AS
C1 - 100%
C2 - 100%
S1 - 100%
AS in Maths = A (AVG SCORE: 100%)
A2
C3 - 89%
C4 - 89%
S2 - 100%
A2 Grade - A (AVG SCORE: 93%)
Student 1 wouldn't get an A* for A-Level Maths, because you must achieve 90%+ on C3 and C4 (hardest exams of Maths A-Level)
Student 2
AS
C1 - 80%
C2 - 80%
S1 - 80%
AS in Maths = A (AVG SCORE: 80%)
A2
C3 - 91%
C4 - 91%
S2 - 80%
A2 Grade - A* (AVG SCORE: 87%)
Even though they achieved an average percentage of 87%, they'd still get an A* for getting 90%+ on the two hardest exams, it's pretty much the same for all subjects (the hardest exams that is). So it's still possible to get an A* without actually getting 90%+ in A2.