Are you using a skimmer?
If so check how much water may have been pulled by the skimmer.
I have mine plumbed to a collection bottle and occasionally my skimmer will act up and fill the bottle, so when that happens my sump can lose almost a quart of water, BUT not from evap, so the salinity doesn't alter.
That may be what is happening and then you add fresh water whenever you didn't need too and get a corresponding drop in salinity.
I now keep 3-5 gallons of fresh and
salt water on hand for just such an occurrence.