Good question...I'll explain the grammar below but first the meaning: It means they collect something unusual because they want to seem like a more interesting person than someone who collects something more common (like stamps for example). You're right to query it because the grammar seems wrong... Really we should use the pronoun 'themselves' to refer to a group, and in this case it looks as though the pronoun 'themselves' is replacing the pronoun 'someone'...which is singular. Is there a previous sentence perhaps? Something like "People who have unusual hobbies..." - in which case 'themselves' is replacing 'People'.