Thanks, Wendy. I gottit...

Just a small point, Pieter: because this is the continuation of his answer in Unit 28, 'Oh, that's very simple', the word 'But' doesn't really make sense here in English.

He would probably say something like: It's because you're a civil servant in the National Education system!
(remembering also the contractions in spoken English to make it sound more natural and less formal)