Internally Dragon uses a rating system, where a difference of one Go rank is equivalent to a difference of 100 points (or Elo points).
The format with kyu and dan ranks, e.g. "3 kyu (-5%)", is used because it's easier to comprehend. The Elo points are shown in the tooltip when you hover over a rating value, that is linked to the users rating-graph, e.g. "Elo rating 1795".
 
Example:
6 kyu ( 0%) = 1500 points
6 kyu (-48%) = 1452 points
7 kyu (+30%) = 1430 points
7 kyu ( 0%) = 1400 points