It could be based on the lubricant's resistance to thicken when chilled.
Lubricants with high VI's have that attribute.
Therefore.....a single grade engine oil that is blended from a high VI base stock, may resist thickening when chilled well enough to pass an xW cold flow and cranking test.
It is quite likely that a mono-grade SAE 30 with a VI of 115-125 could easly pass a 20W or 15W cold performance test allowing that oil to be sold as a 20W30 or 15W30.
An SAE 20 engine oil with a high natural VI may pass a 10W cold flow and cranking viscosity test, but is not declared or labeled as a 10W20.
The hoopla (there is that word again!)about group III mineral and PAO synthetics (I give)
is that those base oils resist thickening very well when chilled.
It is possible for a straight weight synthetic, (by any other name), to pass a very cold performance test.
Straight weight "synthetic" SAE 50 becomes 15W50, SAE 40; 5W40, SAE 30; 0W30 and so on, without the use of VI improvers in the brew.