One of the reasons I think your teacher might have said that is because in persuasive you tend to only really focus on one side of the prompt, which means you neglect a lot of potential complexities and ambiguities that you could have discussed had you written an expository essay for example. So if you really feel you are good at persuasive essays, then to be extra safe, as your teacher says, you would probably want to frame your persuasive essay in an imaginative context which would enable you to address the other side of the issue somehow.
But I suppose, yes, theoretically if you went into the one side you are arguing about in excruciating depth, and wrote it really well, then you could get a 10/10. It's just that doing one of the things mentioned above kind of reduces the associated risk I guess.