What does "half a car short of straw" mean?

In the past, the amount of wheat straw used to repair roofs in rural areas in the north was basically fixed. If the wheat straw was "half a car short", the roof would leak rain. This word is used to describe people, which means "the brain is incomplete, things are always different from ordinary people, and things are always wrong". In Yao Xueyin's novel of the same name, it is used to describe a person who doesn't do decent things and doesn't pay attention to his own image. In fact, this person is quite good. The novel is first suppressed and then promoted, and it is very vivid.