Che cosa significa Writing graders are very rushed and must decide which of seven rigidly numerical ratings to assign to your essay. ... , and wander somewhat off topic. ... What it cannot fail to do is ... ?

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What I don’t understand is the meaning of the whole first sentence and “wander somewhat off topic” and “What it cannot fail to do”. Here’re the details.

Writing graders are very rushed and must decide which of seven rigidly numerical ratings to assign to your essay. This means that it is more important to mimic the most obvious elements of good writing than it is to write an essay in which the content and language actually are good. A high-scoring essay can contain a few grammatical errors, use simple sentence structure, and wander somewhat off topic. What it cannot fail to do is to create the illusion of an organized essay for a hurried reader.