The Citric Acid Assay Kit  reaction is a negative reaction and therefore it starts with a high absorbance of ~ 1.5 and as the reaction proceeds the absorbance decreases therefore A2 will be expected to be lower than A1 for samples containing citric acid.  The more citric acid present in the sample the more the absorbance will decrease.  The change in absorbance (even though this is a negative change) between readings A1 and A2 corresponds to the amount of citric acid present in the sample.

Product Page (K-CITR)