There's no difference. Here's probably why your answer was marked wrong.
For each exercise, there are a number of answers that are validated by the moderators when they add it to the course. Of course, they can't always think of all the details like apostrophes and such, and over time some users will come up with acceptable formulations that are not on the list. If you report the sentence using "my answer should be accepted", at some point a moderator will check your answer, and if it is correct they will add it to the list of accepted answers.