The Drome Score is, for example: If you die during round 80, your drome score will be 79. This is also the first parameter in the rankings.
.Second parameter is players level: P1 - Level 800, Drome Score 85 / P2 - Level 900, Drome Score 85. Both players have same Drome Score, so the player with less level will be higher in the rankings.
Once a rotation is over (Every two weeks on Tuesday), characters receive drome points equal to their drome level times 10 (e.g drome level 80 = 800 drome points). In the Highscores menu at website you can see Drome Score, and that number is your acumulated drome scores (e.g if you always get 80 drome score, then: 80+80+80...)