It's because Trades are in general, lazy f@ckers. They want the easy route out. Unless you specifically specify otherwise. Of course, why not? Time is money.
I had similar.
I'd hacked off all of the loose plaster and asked them to bond out level and then skim. I came back to find that they'd done exactly as you have experienced. They'd simply plasterboarded over the whole lot and then skimmed. I'd lost two inches.
Bloody fuming, I worried that my new kitchen design would no longer fit and the door would have to be smaller (could I now fit a washing-machine through?) Luckily, it just-about worked out still. My doorway was 605mm wide!
Lesson leant. ALWAYS be on site with trades around and keep popping your head in on them from time to time.