I'd say invisibility/erasure might be due to the extreme variety that bisexuals come in when it comes to attractions. It's not often that it's an even split or 50/50 male female attraction, and then there's sheer numbers against you. You're more likely to find a straight/bi man as a woman and a straight/bi woman as a man, even though you're attracted to some degree to both.

That's not to say there aren't bi people who overstate their attraction to the same sex. I have a friend who talked about how 'gay' she was (she's bi), and how much she adores women, but she has only once had a brief relationship with a woman (like a few days before they broke up) and never tried to find dates with other women, despite her insistence that she's more attracted to women. She's in a relationship with a man, but still insists she's more attracted to women. Yes, I know numbers are not on her side, but she has not shown any interest in looking for women on dating sites or trying to meet them through groups/meetups. You can see why sometime people might mistake bisexuals for just straight people, especially if they've known them for years.