It was possible back then because the highest grade was A which could be easily achieved by anyone because you had to get 80% of the mark. But, with the introduction of A*, which was aimed for better discrimination between good and outstanding students. For this, you've to get 90% in the A2 units. This new grading system was imposed because there were a lot of students getting A's and that too with many A-Levels. This aims to bring a standardization like the IB's because IB's allow 6 subjects only. Therefore, it is very tough to achieve an A* with many A-Levels. I personally took five A Levels but had to drop the A2 of my fifth subject because of the huge pressure of getting A* at A2.