After I bought a second weight scale, making sure it works accurately, I got home, and noticed I shouldn't have put any bascula on carpet to measure. On a solid ground, both weights measure up to expectation, a 170LBS.
In theory, I gathered that the carpet increased the force upon the weight because it has potential energy in its elastic structure.
For experiment, I'd need two springs, one on top of another and put a weight upon the top, comparing the spring in the middle by observation with another case where there's only one spring involved. Both tests set the springs upon solid surface.
Now I end up with 1 extra bascula. I should sell it or else give away for free on craigslist.