The problem is that there are years where we have more than one chancellor (namely 1974, where we have Walter Scheel, who was vice-chancellor and acting chancellor in that year and Helmut Schmidt who succeeded him), see the the response for this query here.
The question goes like this:
https://github.com/q-wiki/q-wiki-server/blob/8a9ac139e2cd9db0e5ce8cc72c218b63474b389b/WikidataGame/WikidataGame.Backend/Migrations/20190710215550_AddHistoryCategory.cs#L128-L142
The problem is that there are years where we have more than one chancellor (namely 1974, where we have Walter Scheel, who was vice-chancellor and acting chancellor in that year and Helmut Schmidt who succeeded him), see the the response for this query here.