Yeah, it's confusing . . . <img src="/images/icons/frown.gif" alt="" />
There are some categories that due to their nature are unsuitable for beginning editors to start out in. Oftentimes these are categories that are very large, have a lot of spammy submissions, etc. We try to turn of the "Become an editor" signs in these kinds of categories whenever we can. The letter that you got is one that meta editors send to applicants they think are qualified to become an editor, it's just that they applied for one of these categories.