Your comment supposes that the column headers are the inputs. However, I used the definition from the article-- "Here the numbers along the left edge, labeling the rows of the matrix, are the inputs to the gate..."

So, taking the columns as the outputs, my point is that adding down the columns corresponds to no particular causal relationship in the physics of the gate.

Your comment did, however, make me realize that I hadn't modified the row probabilities for the second row to sum to one (11-month old under foot). So thanks for that.