Early on, and knowing that high VI was "good", chemists started to find additives that are called "Viscosity Index Improvers".
These are typically polymers (think of a white plastic pencil eraser, that's similar) that have a solubility change with temperature. They start "coiled", at lower temperatures, then "fluff" up at higher temperatures. When they do that, they start to interact with and impede the oil, and increase it's viscosity.
But the problem with these molecules is that they are also susceptible to the shear rate within engine components...when components are moving fast past each other, these polymers start to flatten out, and lose their effectiveness at increasing viscosity.
Here's some tests from a main bearing in an engine that shows what Shell were talking about in their Left Hand Columns.
As the shear rate (RPM) increased to a certain point, the lining up of the polymers meant that the oil behaved as a thinner oil, closer to the basestock without VII additives...offering less protection from parts contact.
Because of this behaviour, and the fact that these multigrade oils weren't offering the protection that their grade suggested, HTHS (High temperature, high shear...150C, and 10^6 shear rate, quite typical of a big end at a reasonable RPM) was included alongside the grade as a minimum. The minimum was (naturally) lower than was typical for a monograde no VII oil.
e.g.
SAE30, which would typically have an HTHS of 3.5, was given an HTHS minimum of 2.9.
SAE20, which would normally have an HTHS of 2.9, was given an HTHS minimum of 2.6.
SAE40, which would normally be 4.3+ HTHS was split 0W, 5W, and 10W 40 (passenger grades) were given a minimum of 2.9, and 15W, 20W, 25W, and SAE40 (heavy duty grades) given a 3.7 minimum.
The 40 grades were fixed in 2013, with 3.5 and 3.7 targets respectively
