When the frequency of individual item (cell count) within the list is considered, every combination of individual medical product and individual adverse event is counted itemwise by searching through every list. The marginal cell count of individual drug (D) and that of individual adverse event (AE) are also obtained by counting events itemwise.
Notice that a list contains typically more than one items, and therefore, contributes multiple cell counts. Consequently, the marginal count tends to get smaller than the sum of respective cell counts, and the total count of reporting events becomes substantially smaller than the sum of all the cell counts. See more detail at Counting formulas.