I would guess that your calcium is usually lower than it is right now then. If you raise your kh with your calc as high as it is, you will get a snow storm in the tank. Did you change salt mixes recently? Some are higher in calcium than others and batches can even be different.

This is a good article about ph, and how calc and alk affect it.

http://www.reefkeeping.com/issues/2004-09/rhf/index.php