That looks odd to me, it first uses output_tuples in a formula, then overwrites the value with a new value. Should these lines be swapped?
IIUC it is correct: the additional total_cost comes from processing every output group to check whether it is qualified - since every group is checked the incoming output_tuples from the prior grouping is used. The side-effect of the effort is that the number of output_tuples has now been reduced to only those matching the qual - and so it now must take on a new value to represent this.
