probablyt from the fact that i am not adding the exact, down to the millileter amount that has evaporated
just topping off, till it looks good, and then i double check my salinity, i jsut guess at the level in the tank, i have no auto top off, so a 1/4 in or lesscould make a difference, especailly after 3 or 4 weeks(salinity does not move fast, maybe every several changes it is noticable, and still not even a 1/2 pt on the hydrometer)