The Urea/Ammonia Assay Kit (Rapid) 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 ammonia.  The more ammonia 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 ammonia present in the sample.  This is also the case for A3 values which will have a lower absorbance reading than the preceding ones.  The change in absorbance between readings A2 and A3 corresponds to the amount of urea present in the sample.  A full description of the calculation required for each analyte is given on pgs. 6+7 of the data booklet.

Product Page (K-URAMR)