Std queen is 60" wide.
Std king is 76" wide.
Std Cal king is 72" wide.
I can see leaving a little mattress unsupported on each side, but 6" or 8" per side is ridiculous IMO. I think someone screwed up, but it won't surprise me if the factory says "well that's how we do it".
It will ruin the support in your mattress. So if it comes down to you fixing it yourself, adding an overhanging layer of plywood is advised.