The way I've seen it used is just bisexuals indicating that they strongly lean in one direction. I don't see anyone pretending this is anything but a form of bisexuality. I'll let the bis speak for themselves here but just because you're bisexual doesn't mean you're simultaneously interested in both sexes at a given time or equally interested, and this is one way to indicate that. It makes sense to me that bisexual people want a way to differentiate themselves. Someone of a particular sex with a strong preference for the same sex is not going to have much in common with and will want to disambiguate themselves from someone who only dabbles casually with the same sex.