You must remember that errors are a result of a combination of factors. They can be unintentional spelling mistakes or grammar errors. Since grammar corrector identifies errors based on predetermined rules based on grammar check rules, and previous submissions, it will not flag a one-time error. For a new grammar check rule to be created the error must have been made more than once and by different users. Let's take an example, the sentence 'our base is more than two clicks to the north' will not be highlighted for errors. People who are unfamiliar that the word 'click' is used to indicate miles in modern military terminology, the sentence will be meaningless. None of the English grammar check online applications will identify this sentence as incorrect. Why? There are no spelling or syntax errors in the sentence.
Even if a sentence comes across as bizarre, but the syntax is fine, grammar corrector will not suggest any corrections. You can test this sentence in any English grammar check online application, and none of them will identify any mistakes in it. Take another sentence: 'the oranges eat the people'. Although grammar corrector is very sophisticated, it's not human, and it won't flag this sentence either. In fact no English grammar check online application will flag it. However, should the sentence be: 'the oranges eat the people,' error on the verb 'eats' will be highlighted.
Grammar corrector is comprehensive spelling and grammar check application. It does checking based on predetermined rules which have been programmed into it. If a sentence does not make sense to the human reader but contains no spelling or grammar mistakes, it will not be flagged. The grammar rules used by grammar corrector are updated as and when they are formed. Like other spelling and grammar check applications grammar corrector highlights only those errors which contravene grammar rules.