Here's a link that was posted for me last week, good explanation of Calcium and Alk...
Calcium and Alkalinity by Randy Holmes-Farley - Reefkeeping.com
If the reading is really that high, I would guess it has something to do with your tap water. Tap water can have all kinds of nasties in it that you don't want in your tank. It also makes keeping the chemistry right difficult because it already has dissolved salts in it (possibly including calcium). That's why its best to start with R/O water, then you're getting the best mix when you add salt. You shouldn't have to add buffer to your new saltwater, everything should be pretty well balanced when you mix the salt in, however, if the pH is low, then something in your tap water may be bringing it down.