Many dragon powers use a system called ‘scaling’ for calculating their bonuses, meaning that the first four ranks give a bigger bonus than the later points.
For example, the Fires of the Volcano power gives an immunity bonus of +5% for the first rank, +4% for the second rank, +3% for the third rank, +2% for the fourth rank and +1% for the fifth and later ranks. This would be described as a +5/4/3/2/1 scaling bonus.