赞同千千老师的解答。我从另一个角度补充几句我的看法。首先根据《牛津英语用法指南》的观点，作者认为than 很像关系代词，可以代替从句的主语。

因此，英语中实际存在两种说法，即than usual 和than is usual. 这两个意思相同，只是后者更正式，前者更简练，更常见。

如果从解释than usual 的角度来说，也勉强说得过去，因为than 后面省略了主语，该词典的作者犹如“补全还原”省略的要素。

1. It is usual that he is not late. 按照现行语法规则一般认为是错的，特别是英国英语。但在美国英语中，特别是学术文献中，it is usual that 这种句式似乎经常出现。图书语料库的例句很多，我仅提供几个外刊例句供参考：

2. It is usual that the last section of a share purchase agreement consists of general provisions sometimes referred to as 'boilerplate clauses'.

Joanne Cox, Business Law, p 172

3. It is usual that the media conference will begin with a welcome to the media...

Matthew Nicholson, Sport and the Media: Managing the Nexus, 2015, p. 205

4. Thus, it is usual that mathematicians and students of mathematics concern themselves with being able to transfer planar calculus to the computation of lines and normals on a surface.

Phil Dyke, Two and Three Dimensional Calculus, 2018, p. 120

以下各句也出自英语出版物，来源省略。

5. In reality it is usual that what worsens the conflict is "the escalation of violence".

6. In these cases it is usual that the government analyzes the company's accounts to be able to regulate it, since it isn't the same for the company to be making profits or losses.

7. It is usual that the head should speak for the whole body, and what the head says is considered to be said by all the rest.

8. It is usual that teams will discuss what exactly would be needed in terms of an engineered solution to the failure mode. 

第1个，如果不细扣什么是省略，仅从帮助理解的角度来看的话，我觉得这个解释可以接受。

第2个是把频度状语按外加来解释了。不行。