Superking duvet measurements!?!

[Emeraldeve]

Hi all, I have recently purchased a Dorma full forever Superking duvet to replace my king duvet in the same brand and type. I had no trouble with with king one and it was true to size. However after putting the Superking duvet into my duvet cover there was quite a substantial amount of material spare (more than your average.) It looks really odd so I measured it and found the problem. My super king duvet was 240cm wide approx instead of the advertised 260cm approx. I took it back to the store and bought another and lone behold I encountered the same problem again. I really acouple do cms here and there is to be expected but I didn’t expect it to be undersized by almost
20cms

I would be ever so grateful for any recommendations of ‘good’ (nice, thick marshmallow like) manmade duvets in a 13.5 tog that are true to 260cm size?
Thanks