Where to add Carbonate in water profile

The carbonate content is taken into account by the Bicarbonate value which is the active form of Carbonate in the water. 
Then why is the Alkalinity in BS3 so much lower than in BruN' Water?  BruN' Water has you add in the carbonate as well as the bicarbonate.  When I start adding in my carbonate number to bicarbonate in BS3, then it begins to start to match the BruN' Water numbers.