The easiest explanation would be evaporation. Salt doesn't evaporate with the water, so the concentration rises.
The other possibility would be supplements. Calcium Chloride will cause the salt concentration to rise over time (it's the calcium part of 2 part) so if you're using something like that or purple up, or other things like that...then this is most likely your culprit.
How big is your tank, what's your evaporation like, and what kind of rise in salinity are we talking about?