Perhaps their teaching has been poor? Many people won't understand topics unless they are outlined to them specifically by a teacher. That's actually the case for me, and it's why I've found YouTube channels like ExamSolutions and EconplusDal (and PajHolden, of course) so useful. They're far better than textbooks or the revision sessions that my Sixth Form puts on, which is to just go through past papers in Maths (no explanation of the topics, so no one turns up, but Maths teachers still get annoyed at us).
Like you say, though, I don't understand why people aim for anything lower than a C. I heard someone today saying, "I'm just aiming for a D so I can get back into Sixth Form next year." I think some people could do better at other establishments, like colleges or in apprenticeships, but such places often receive stick so their worthiness is lowered, and thus people feel they have to go to Sixth Form. There are over 400 people in my Sixth Form, and that's due to a mix of the school lowering grade requirements (some people got into Geography with a D at GCSE), adding extensions to the centre (two currently ongoing), and an increased range of subjects. And that all leads to class sizes getting far too big, so teachers can't focus on individual pupils as much as they once could.
Sorry, that sort of turned into a rant.