我在读完这句话后，感觉than they appointed 语义不清楚。appoint 是个比较正式的用词，而且仅具备及物动词的功能，没有不及物的功能，所以我觉得它应该加宾语。如果仅用they appointed，听话人会感到怪怪的，appointed what? 任命的这个人也存在不是一个candidate的可能性，这里没有宾语含义模糊，所以加了 the one，说话人和听话人应该都知道the one 指的是谁。
前面那几句不同，语义非常清楚，也符合惯用法，在非正式文体会出现，所以作者没有确认为错的，仅是从语法角度怀疑。
1. This result is better than ___ would probably be achieved by a vaccination policy.
achieve 在比较分句用于被动语态时，当它的主语为比较对象时多省略。但不是所有动词都可以这样用。
例如：This is far more than has been achieved under self-determination in the United States.
2.The price was higher than he wished to pay ___. 这个句子语义是清楚的，pay 的对象只能是price.
3. When children start school they tend to get books that aren't as rewarding as they've had ___.
have had 后面应该指的是书，所以不会产生歧义。
4. The eastward movement of the Atlantic thermal ridge was forecast to be a little less than ___actually occurred.
than occurred 也可以用than what occurred. 都属于常见搭配。含义明确，指实际发生的 movement。
以上是我的理解，不一定对，仅供参考！
