Board Thread:Questions and Answers/@comment-223.27.94.3-20131012185849/@comment-24027950-20131013000753

To expand on Hina sama's explanation, calling someone by their family name is a polite way of refering to someone in contrast to being "close" or "intimate" friends (whereas they call them by their surname.) Usually you would call someone by their family name when you don't really care much for that person or is someone you don't know as to show respect. Kind of like calling your friend's parents Mr. and Mrs. instead of their first names.

And the reason why Hinagiku was doing that was as Hina sama stated (kind of saying we are "not friends" in a way I guess.)