That's Just tolerance of a Narrow Range in a Class of viscosity, since they are mostly spread from each other in units 10 magnitude "weights", on the operation temperature side (not the Winter side).
J300 used until recently SAE 10, SAE 20, SAE 30, 40, 50 and so on. For example, THERE'S NO SAE 21, 22, 23, 24, 25, 26, 27, 28, 29, so there's must be a range inside a class (20 in this case) separating a class from other classe to the next.
Now you see SAE 8 and SAE 16 and a new revision comes ...