Two numbers do most of the work in describing a crude: API gravity and sulphur content. Together they place a grade on a map that largely determines what it is worth to a refiner.
API gravity is an inverse density scale. Higher API means lighter crude, which yields more of the light products that historically command higher prices, gasoline and naphtha and jet. Lower API means heavier crude with more residue, which needs additional processing to become anything valuable. Sulphur content splits grades into sweet, conventionally below about 0.5 percent, and sour above it. Sulphur has to be removed to meet product specifications, and removing it costs money and requires units not every refinery has.
The value of light over heavy, and sweet over sour, is not a constant. It is set by the product market and by how much upgrading capacity exists. A world with plenty of coking and desulphurisation capacity narrows the discount on heavy sour grades, because more refiners can process them profitably. A tightening of marine fuel sulphur rules widens it, because low-sulphur output suddenly commands more.
A grade's differential is thus a fitted quantity, not a physical constant. Somebody has priced this grade's yield against the benchmark's yield using current product prices, netted freight, and arrived at a number. When product prices move, the correct differential moves even if nothing about the crude changed at all.