Kinda weird where your Alk went down so much and Cal is still high. From my understanding alk and cal usually move together unless you have a partial nitrogen cycle. Based on the following article.
http://www.advancedaquarist.com/2002/11/chemistry
"The vast majority of alkalinity depletion in most tanks also comes about by the precipitation of calcium carbonate, as described above. In this process, as alkalinity is depleted by 1 meq/L, calcium will be depleted by 20 ppm. There are some other processes that can lead to alkalinity depletion, including partial cycling of nitrogen (from organic compounds to nitrate and no further) and the incorporation of magnesium into calcium carbonate, but these are generally much less important than calcification."