My opinion is pretty much inline with Cryssoberyl. Would like to read more stories with that dynamic.
The title is somewhat awkward though, I don't know how anything in particular could be "less" than a boyfriend, lol. The heteronormative connotations...not something I particularly care to see in a yuri manga.
To me it just sounds like the title was too literally translated.
I figured that it was just an odd translation as well. If there was a heteronormative connotation it would be something like "more than a friend (that is a girl), less than a boyfriend".
Wild mass Guessing:
A common belief amongst very traditional Japanese people is that women are inherently less valuable then men, so the title may connote value, someone so important their more important than a regular girl but still not a boy.
Another possibility is that it means "girlfriend" in the platonic sense, like how some women/girls say they "have their girlfriends over" when what they generally mean is that they have their friends over and not that they're in a polygamous lesbian relationship, in which case we come around to the heteronormative accusation once again.
Or it could be a translation error. Or just a really bad choice of words on the authors part. We really can't say for sure unless we got the author to comment on it.