ok. physical integers need ranges or tolerances.
the ideal – the unit – is a vessel containing an unknown substance and has a weight which is conferred upon it by gravitational forces on earth.
since the gravitational forces are constant here from our perspective generally around the planet, two similar vessels engineered can contain the same weight of the same substance even if contents settle.
when weights are added in a linear fashion they work as arithmetic is expected to work.
note: i use container because a manufacturing mold is a container also.