I bought some sand a few years back at Exotic aquatics. The stuff WAS NOT aquarium sand. Not sure what it actually was, but my calcium stayed at 900-1200+ all the time for a few months after I got it. I tried 50% water changes every third day for a week, only getting down to 650, but by the next day it would rise back up 30-40 again. As soon as I changed my sand to Aragamax Bahama oolite (which IS aquarium sand), everything went back to normal. The only thing good that came of that sand was I learned about KH via that issue! Reason being, If your KH gets to low? Some sands can dissolve into Calcium.
I'm not saying your sand is what I had (god forbid). However if you have drops in KH, causing PH fluctuations it can sometimes cause sand to turn into excess calcium, thus filling in the missing Carbon with Calcium.