[Archonsod]

When did you check the graph? From what I can figure, if you grab a VP location after summer, you won't get anything from it until the following summer (when all nations holding VP locations are credited). If Yomi was the only nation still holding VP provinces during Summer, it would be the only nation gaining VP's.
You can really irritate your opponents by raiding their VP provinces only in late spring, thus denying them the points for holding the province the entire year...