It is because of using absorption costing. If you are unsure you need to watch the absorption costing lectures for Paper F2 (because this is revision of Paper F2).
I do explain the lecture, and I can’t repeat the whole lecture here. However, it is unlikely that you would be asked the fixed overhead variances in F5 (it is in the syllabus which is why the lecture exists) but again is unlikely.
If you really want to understand the reason then you need to watch the lectures, but frankly because of what I wrote above I would just learn the rule.