I reread the comic in question and yeah I think its badly worded. It seems to be sort of like one of those life drain spells in D&D that gives you temporary hp in that the target of the spell loses those hp NOW but regains them over time. If succubi actually permanently stole max mana capacity from people there would have been a holy war ages ago to eradicate the entire species. No spellcaster would be willing to let them continue to exist no matter how hypnotic their cleavage. Im guessing what they do is a temporary max mana debuff that goes away after a day or two so long as they dont drain all of it (hence staying in the positive) so it causes no long term issues to anyone. At best it might suck to be the local battlemage the night before the final fight starts getting your pipes cleaned only to learn it was a succubi and you are down to 80% max mana till the day after tomorrow.