I would be more interested in why someone who is bisexual would want to call themselves gay, if they are not ?

In most cases, I suspect, because they don't actually realize they're not: bi invisibility can make us invisible even to ourselves.

If (like the majority of people) they think bisexuality doesn't exist-- or have just never heard of it-- and are a Kinsey 4-5, then they'll likely "round themselves up" to Kinsey 6. It's either gay or straight, after all; gotta pick one!

And even if they DO think that bisexuality exists, they still might have gotten it into their heads that this can only mean 50/50; anything else doesn't count. So you're a guy who's a bit more male-inclined, or a woman who tends towards the ladies? Gay. Even though you're aware of being attracted to both sexes.

Also, as per the common bi-erasing "rule" that sexual orientation = whether you're currently dating a man or a woman, they're paired up with someone of the same sex, so... gay. (Again, regardless of being into the opposite sex, too.)

In short, bisexuals often get monosexualized... and it's not always in the hetero direction.