Why is my NaCl solution seemingly saturated, when I followed the recipe for an isotonic solution?
Salt added to food could be sea salt, common table salt or iodized table salt.

Common Table salt typically contains anti-caking agents (desiccants) such as magnesium carbonate, calcium silicate, dicalcium phosphate, tricalcium phosphate, calcium carbonate and sodium alumina silicates.

Iodised table salt may contains added potassium iodide for people who may otherwise have low consumption of this nutrient which is essential for proper thyroid function. Since iodide will slowly oxidize in moist air, stabilizing agents such as sodium thiosulphate or dextrose are also added. Some will also contain a base such as sodium bicarbonate, disodium phosphate or sodium pyrophosphate to also inhibit oxidation of iodide. There may also be added sequestering agents which bond to trace metal ions present as impurities to prevent them oxidizing the iodide.$^{[1]}$

The presence of magnesium and calcium will add "hardness" to the water which could cause precipitates such as calcium carbonate and magnesium carbonate to form.