OP, I want to thank you for starting this thread. I'm another one who had no idea that king size duvets were not exactly square.
To answer PPs about how so many people have no idea how to put on a duvet cover - I learned to do this with a single duvet which quite probably had stitching lines running lengthwise, and then with a double I followed my imaginary (and incorrect) rule of stitching lines running head to foot. By the time I upgraded to a king size, I didn't think there was anything more I needed to learn. I just assumed it was a failing of my fairly low-budget bedding that nothing quite fitted exactly.
It's been mentioned that king size is 230x220 cm, but having checked several brands the standard sizing seems to be 225x220 cm. I'm sure I would have checked for squareness at some point by folding mine into a triangle, but for a large and bulky winter duvet with slightly rounded corners, a 5cm difference would lead me to the conclusion that my folding was slightly off and the duvet was square (because nothing else would make any sense), and not that it had been manufactured with one edge 5cm longer than the other.
It's such a stupid difference, because it's just enough for it to be ill-fitting with the sides empty of duvet and the top and bottom ends very slightly bunched up, but not so bad that it's immediately obvious what's wrong.
Anyway I have now enjoyed a night's sleep where the top edge of the duvet was not annoyingly bulky, and I could pull the duvet across by grabbing a plump edge instead of a sad bit of empty cover. Thanks OP!