^Yeah that's pretty up to every man themselves to say what songs counts as.
She have many songs that didn't got into albums though. Like Mirrorcle World or Ever free but they still counts as her. Even though Dream ON isn't counted as her "51th" singel, it's still listed on her discography on her official website. It make me think, it's the latest single "with" her... Aw... 2010 looks like a long time ago now.