While keeping the fish at 1.021 does save a little money on salt, the main reason fish stores keep their salinity in the 1.020 range is that is the salinity that fish are shipped at. Lower salinity reduces the toxicity of ammonia. When fish are shipped from the collection sites around the world to the wholesalers here in the US or Europe they are shipped in lower salinity so the ammonia that builds up in the fish's bag is less toxic and the mortality rate is much lower. Wholesalers do the same thing, they keep their fish in water that has a salinity of about 1.020 and then ship it to the local fish stores in low salinity water. Your local fish store thus keeps the water at 1.021 so they do not need to acclimate the new fish that they are constantly bringing in to their stores.