ASTM D2270 Viscosity Index Formula/Table derivation
back when it was invented they measured all the oils they could find.  They got the worst (most viscosity change) and the best in each group and ranked these as 0 to 100.  All others were plotted on this "linear" scale.

The unit was used for dome time until the best was bettered, then they invented VIE (E is a subscript, and stands for extension, strictly all VI's above 100 are VIE's).

It is a horrid unit because it is far from having any useful meaning, unfortunately it has some significance.