Woolmark connects Merino softness with fine fibre diameter and the ability of those fibres to bend against the body. Diameter is measured in microns, with one micron equal to a millionth of a metre. Coarser projecting fibres are more likely to create a prickling sensation when they press on skin.

Finer Wool Fibres Bend More Easily Against the Skin

That explains why softness cannot be judged from the wool name alone. Yarn construction and fabric finish also affect the experience, but the fibre measurement supplies useful underlying information. A lower diameter describes actual fineness rather than merely a fluffy-looking or brushed surface.