You have it set up so regular members can't set a custom user title and only have the default user title or user title ladder you set for them—for argument's sake, let's say the user title is Member. The only way for them to be able to use a custom user title is if they're promoted to a staff user group, for example. They get promoted, and instead of keeping the default Staff user title by leaving the field blank, they set their own custom user title: Totally Awesome Staff Member. A few years later, they become so busy with their lives that they don't have time to be staff and file a resignation. They are then demoted, but their user title doesn't reset to Member and instead they still have the custom user title they set: Totally Awesome Staff Member.