Because there are other factors that determine your position in ranking list. It may be amount of damage you have done, or number of attempts … I don´t really know what. But for some reason, if we have all done the mission without taking dmg, however we have different positions in the ranking (and different amounts of coins in reward). It shows much more the first day.
Therefore, it´s possible that those who go in the first 3 places have received damages like all of us, but for some reason they were placed first.
But we must also take in consideration other detail, we don´t all see the list in the same way.
*Common sense indicates that perhaps damage you have done and damage you have received, is subtracted. And that determines your position. And, if that were the case, we must bear in mind that weapons do not always fire with the same hit power.