Good question!
Your struggle to keep salinity stable is because you aren't using an automatic top off system. It's like this...
Water in your tank evaporates. And the rate of evaporation is dependent on the temperature and humidity in the room. Evaporation rates during the winter are higher because we heat our homes and the air in them tends to be dry. In contrast, summer tends to be more humid, slowing the rate of evaporation.
But while water evaporates, salt does not. Salinity is a ratio of water to salt. As water evaporates, the ratio of water to salt increases. When we add water, the ratio goes down. This explains the fluctuation in your readings.
By having an automatic top off system, water is added to the tank as it evaporates, keeping your salinity fluctuation undetectable.