A lot of schools have a default option as four AS then 3 A2, or four A2 if further maths. But then I saw literally everyone who wasn't a strong oxbridge candidate ended up dropping their fourth A level in the further maths set and a huge proportion of people who initially wanted to keep all 4 and did other subjects (3 sciences + maths) ended up dropping it too. It's just a method of showing you have good time management which is a key skill Oxbridge students require unless they want to drop out of get a third or lower.