I would guess it's the ages of the guild members. I have 2 Iron Age cities, and they both are in solo guilds and get 1 Guild Power Point for each Crown level. I have 2 CE cities that are in solo guilds and they both get 53 Guild Power Points per Crown level. And my OF city is in a solo guild and gets 117 Guild Power Points per Crown level. If I remember, I'll check those numbers next week to see if they change.