IMVHO, even a straight weight like "30" has a W low temperature viscosity value. But maybe the low temperature viscosity measurement is somewhat variable for that stuff and the oil company doesn't want to specify it. When they learned to adjust the "W" viscosity and get consistency, they certified dual weight such as 10W-30.
So when there's a single number straight weight in motor oil or gear lube, they are just not choosing to specify a low temp viscosity. They don't want to say 10 +/-5W 30. Just guessing.
When I first started driving in 1960, I remember being puzzled at 20W-20.
Read the following and you won't want to talk about oil viscosity again. Scroll down about half way to see a cartoon about mineral vs. synthetics. I've got a problem with how they show "larger." The bigger ones are actually longer and not bigger in diameter.
https://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm
