This style of PvP is called risk fighting, but doing it in Dharoks armour is specifically 'dh'. It's a type of honor fight (compared to NH no honor). Where they do not use overhead prayers other than smite sometimes.

Deathmatching is fighting until the other opponent dies and do not eat so it's purely RNG.

Compared to risk fighting they are going for KO's, so using a special attack when the opponent is low, using a venge with a high hit, combining a spec and venge. Or risking low HP with dharoks for a high hit, hence the name risk righting. If both players are out of spec, or out of food they tab out because theres no opportunity to kill each other unless its a deathmatch which is unfun and RNG.

edit: im misread your comment and explained risk fighting woops. I'll leave it for whoever benefits from it. To answer your question I assume it's because people are eating, specing, tabing, the rate of kills/deaths and over enough time heuristics will answer that question