It seems like as if they think the the term Mane 6 is literatly cannon dispite it being a creative name done probably by a fandom. (If it was called Main instead of Mane, then there would be no excuse to call it Main 7)
I'll never understand how anyone can genuinely believe that, or why. These two have been friends for life, far before the mane six. Of course Twilight cares deeply about her new friends, but in no way more than the one who's known her all his life.
It's like a lot of fans thinking the two are like brother and sister. The thing is, they never really refer to each other in that regard. Heck, I don't think either of them said that they love each other. I'm guessing the writers are going for the subtly of the two showing their appreciation for one another through their actions.