Water Report: pre-filter and post-filter

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We use GAC filtration with gravel beds to strip out chlorine/chloramine. I figured if I was sending a sample to Ward Labs I might as well check before and after the filter to satisfy my curiosity if nothing else.

Water was run for over 10 minutes in filtration mode before sample and then we switched over to bypass and ran a few more minutes before the unfiltered sample.

Results;

     			UNFILTERED	FILTERED
pH                     	7.3		7.7		             
Sodium			8		9
Calcium			33		34
Magnesium		9		9
Total Hardness		120		123
Sulfate			6		4
Chloride		16		16
Total Alkalinity	82		108

What could cause this difference in total alkalinity?