It is somehow precipitating out of your tank quicker then it should be. Also I am assuming that you took your water in for testing sometime after the 4th day of a water change? I could care less what kind of salt you are using. If you have a smaller tank, your definetely need to dose - even if you have a larger tank, you definetely need to dose. Buy a cheap calcium test, then 48 hours after doing a water change, check your calc and alk. The directions will tell you what you want them to be around. If at 48 hours it is still around 430, do another check after 72 (then so on and so forth). Eventually you will find out why and when your levels are dropping.
Either that or your salt is NOT "formulated" for reef use. Meaning it does not have higher levels of calcium in it. Calc and Alk also effect PH A LOT. If you can not get them balanced, the PH fluctuates and will mess up one or the other. If you have a larger tank, most people go with individual bottles for Calcium, Iodine, Strontium, Molybedenum and my favorite Magnesium. Magnesium helps PREVENT calcium from precipitating and alkalinity fluctuating. If you have a smaller tank (inside 70), buy a chemical that does all the additives and buffers. Long story short, you need to dose. Some people will tell you that you dont because they have 150 pounds of LR in their tank which releases more calcium carbonate hense keeping their calc levels up.