There any source material on that calculation? 10lb difference is pretty substantial on a 49lb hog, but not nearly as concerning on a 349lb hog.
Just curious, I have no experience on measuring myself.
I don't know the origin of it....but I have weighed many, many hogs and then compared against the Heart Girth method and it remains fairly close for the hogs in my area.
That said, there ARE notable exceptions. Pregnant females (depending on far along) can weigh significantly more than 'taped'.
Certain body types (long slender hogs, versus squared off, chunky hogs) will weigh differently, but on average the mature hogs, in good condition will be pretty close to the posted figures. Close enough that I rarely put them on a scale anymore. In fact, never unless its a really large one....and then just to satisfy my curiosity.
If it appears to be under 250 lbs. I'm not really interested in its true weight. Most of the time I can estimate the hogs weight within 10-15 lbs of what the scale will tell.
After many hundreds, you develop an eye for it.