Only multigrade come with a W number.
Go read the chart above where the cSt for 20W is the same as cSt for 20 at 210.
If a fluid flows at X when cold, and another fluid flows at X when warm they are the same X-speed-viscosity.
Obviously one fluid will not flow at the same rate at two different temperatures so it gets two ratings.
The only reason some bottles only have one number is because they were only tested at 210.