plague is inflicted like any other debuff, so there's a potency check. other than that there are certainly a few variables.
maybe one team is using more special attacks than another (those inflict plague under talzin lead). in fact if you're using nihilus or deathtrooper in your team, that could be influenced by how and when your cooldown increases hit.
maybe one team is inflicting plague right before you take a turn, so you're spreading the plague around sooner.
maybe one team is hitting harder with talzin's aoe, so you have less health for the plague to eat away.