Nurgle corruption dramatically decreases as Tamurkhan, and I dont understand why

Started first Tamurkhan campaign, actually my first serious campaign as a corruption faction, got my corruption to 100 in starting provinces pretty easily. I decided to befriend chaos dwarves, and went south through orks to goldtooth. Around turn 30 I noticed that my nurgle corruption started going down across all my territories, and pretty quickly. In some provinces corruption tooltip looks like this: "+1 technologies, +8 buildings, -28 adjusting provinces". What this "adjusting provinces" mean? Afaic chaos undivided corruption from chorfs should not spread by itself, and even if it does, it only caps at -5 for other corruption types, how can it be as high as -28? My friend was playing as Khorne Challengers, and took Archaon capital, which is a province bordering my capital. My capital only bordrers this one, and my other province, and both my provinces has this "-14 adjusting province" debuff, and my friend's province has no such debuffs to his khorne corruption. I tried googling but found no answer btw.