Along the lines of what Razorbacksfan said...
Think of it this way. If you have a 10g tank at 1.026 and let 5g of it evaporate away, the water in the tank is NOT still 1.026 - it is quite a bit more. Now if you dump 5g of RO
water in the tank, you'll be back at 1.026. But if you dump 5g of saltwater at 1.026 into the tank, then you'll end up somewhere below where you were with 5g, but above 1.026.
If you're only replacing a little bit of evaporation each day, you won't see the rise in salinity even with a refractometer. But if you constantly replenish that evaporation with saltwater over the course of a week, you WILL see a rise in salinity.