Ok, I figured it out. In the descr_region file, Scarn, Neburkha, and Burh Sagath (so far) all have religion values greater than a hundred.
Code:
South-Logathavuld_Province
South-Logathavuld
venice
Rhun_Rebels
131 114 108
mountains, roads, big_city, towers, no_brigands, expand_settlement
5
1
religions { catholic 63 orthodox 0 dwarves 39 northmen 38 heretic 0 islam 0 }
Celduin-Far-East_Province
Celduin-Far-East
scotland
Human_Rebels
167 207 122
grasland, no_brigands, roads, big_city, towers, dale_rivers, no_pirates, coast, expand_settlement
5
1
religions { catholic 37 orthodox 8 dwarves 0 northmen 61 heretic 4 islam 0 }
North-Khand_Province
North-Khand
venice
Variag_Rebels
200 242 226
desert, no_brigands, roads, big_city, towers, expand_settlement, hr_2, hr_d
5
1
religions { catholic 95 orthodox 0 dwarves 4 northmen 3 heretic 6 islam 0 }
So instead of using 100% culture as the max, it uses whatever values each of those adds up to. I'm not sure why that causes huge unrest values, though.