This is correct. The equipment has been designed for speed and not accuracy and is used by people who don’t care and don’t check whether the measurements are valid or make sense.
They simply put the item on a conveyor and the machine measures at high speed.
The laser measures to the extremes of the packaging. So if there is a clear polythene bag, this doesn’t matter, it measures to the edges. If the greetings card is bent or the flap is open slightly, it measures to the edge. If your item is in a pillow pack, it measures to the extremes of the edges.
You get what I mean, I hope.
I am fed up having to change the way I pack items to suit Amazon’s methods. I have to sellotape everything down purely for getting the correct measurements and not be stung for higher dimensional fees. These have already seriously damaged the FBA side of my business and it will take a while for old stock to sell through.