it may also depend on teaching environment for certain subjects, and the type of learning that people want to do.
some may, as said, have more than 3 interests, and they may find that their school (and possibly the course offered) would help them pursue their interest in a focused manner, with more guidance on important topics and problem-solving than self-study or reading solely for enjoyment might give.
it's likely that that would depend on their previous experiences (if any) with the subject at GCSE, the resources they thought they might have, and their knowledge of how well their teachers have taught them.
and, of course, it's also nice to work towards qualifications that keep your options open to a range of professions, and give you practice in a range of transferable skills.