If you are using a swing arm salanity tester, do you rinse it after testing? If not that maight explain what is going on. You fill a bucket, test several times, dissolving salt stuck in the hinge for the float after the 3rd or 4th test. Then you put is away, salt dries and then you go to test again once, and the arm has slight resistance, thus giving it a lower reading.

If you have a skimmer on the nano that might be your culprit as it wouldn't take much volume to lower your salinity.