You have to remember that the answers are being marked by a computer programme. If you think that the alternative answer looks equally natural in the sentence in English, then you should flag it so that it can be added to the possible correct answers. It is only when they get a lot of feedback on a course that it gets refined for future users.We are all part of the process.
Probably considered to be typos. The Hebrew course is reasonably lenient about these compared to some of the other courses, i.e. if you write one letter wrong in a word and it then happens to be the same as another word it is not automatically considered to be that other word, they give you the benefit of the doubt.