It is really a thing. You've been super lucky. After my first apartment, we started hiring cleaning services to do a pre-move-out clean, which, in retrospect, was pretty stupid, because the condition of the apartment's got nothing to do with anything.
It is in most of the UK rental agreements I've had. If they have to hire a cleaning service to clean it up, they recover the cost from your deposit. You're supposed to leave it in the good condition it was when you moved in.
