It's simple, you just have to think about it. Say you lose 10 percent of water each day, and replace it with 10 percent tap water. If you lose another 10 percent the next day, you'll lose some of the tap water added the previous day (in that loss percentage you have tapwater from the last day). The salt never evaporates.
It's kind of like a 50% water change one day, and a 50% water change the next is not a 100% water change. Thats the kind of theory I suppose that I'm trying to get across.
Be ocean minded, keep our beaches clean.