I think what happens is that people will confuse some of the modern "boxcutters",with light,thin blades that extend thru the handle & are broken off as each section dulls,with the "Stanley knives" (as you Brits call 'em),which are also called construction knives in the US,the blades are thick & honed to a 30 deg angle,they're used to cut thru a variety of materials,thick carpet,rubber hose,electrical wire,small PVC pipe,thick leather,etc....
So they'll easily cut thru flesh,as
Van Rijn's unfortunate experience shows.
don't feel bad VR,I've got a few scars myself 