The "Difference Score" is calculated for the top metrics/dimensions when initially built using "Build" in "Segment Comparison". It triggers a job that uses all the statistical methods to calculate the difference score. As a result, the difference score is shown for all the top metrics/dimensions.
However on dragging a new component such as a new metric, the job isn't triggered again. The difference score won't be calculated in those instances.
The top metrics/dimensions that are displayed in the table are derived from the statistical algorithm. They represents the statistically most differentiating metrics between the two segments selected and compared. The row items that are applied after the segment comparison has finished do not receive a Difference Score. This is why n/a is seen for those rows.