There are 2 types of hydrometers and the inaccurate ones are the floating needle style. The float inside the hydrometer will trap small bubbles giving a false reading. Almost always. They are fine where accuracy is not important like the S.G of a car battery. The other type are quite accurate and are the floating bulb hydrometer. The entire bulb floats higher or lower depending on the S.G. Unfortunately I have never seen one scaled for aquariums. The ranges we are interested in are about 1/4 inch or less on the scale. That
Makes it hard to tell the difference between 1.02 and 1.03.
Refractometers come in a variety of scales including the aquarium range.