...you're right! I originally tested the buffs by casting their respective spells once and using a trigger that would count how many buffs of each type a unit had, but maybe I wasn't thorough enough. I just tried casting Unholy Frenzy and Lightning Shield first on an ally, then on an enemy, and it treated their polarities differently for the two casts.
Hmm. I wonder how many abilities are impacted by that sort of logic in ways that we normally don't see? Looks like more testing will be needed before I update that chart.