Ethanol and sugar solutions have an overlapping range of refractive index. This means that a refractometer for sugar solutions can also be used for alcohol measurement of distillates. And to some extent vice versa.
From 0 to 85.7%abv the refractive index increases, after that it decreases again. So some refractive indices can mean two %abv values, which are then both displayed. For example, a displayed value of 70%abv can also mean 96%abv, or a sugar solution with SG 1.084, since the refractive index is the same.
So the calculations are only valid when using a refractometer. Not when using a spindle or other density meter.

General Information about Refractometers