Because more viscous is thinner and
Less viscous is thicker.
Better correct yourself before you have Wayne jumping in here calling you dumb.
Viscosity is a unit of measurement. It’s not relative. It’s going to vary at different temperatures yes, but an inch is an inch and a foot is a foot. It’s a unit of measurement.
If two fluids are tested at entirely different temps their viscosity will be recorded as the same, at those corresponding temps.
Here is a proposal for you.
You measure a random fluid at 50 degrees celsius and it’s rather fluid. So you think it’s rather viscous so you give it a reading of 60. You freeze it at zero and it becomes molasses like, thicker, what reading what you give it?
Keep in mind, the higher the number the thicker.
:pop