I was a lil confused at your post, once salt dissolves in the tank, it *never* comes out-- to my knowledge there is no way a sw tank can lose salinity other than by very, very slow salt creep and the occasional physical removal of water (test kits, acclimation drips, etc....) and topping off with freshwater.
I'm betting you use a swing-needle type hydrometer (I do too). Be CAREFUL with these, they are very, very easy to get a very very bad reading. Make sure you fill it up VERY slowly so you don't form bubbles on the needle. If you have any, it WILL give a higher-than-reality reading. My guess is this is what happened. Try buying a big rubbermaid trash can (20 gallons or so) to premix and age your water in. Water mixed and aged a couple of days is quite superior to freshly mixed saltwater.