I will chime in being a bisexual male age 70.
My early years were definitely all heterosexual all the way up till my late 50s. But I always had thoughts of having sex with other men or boys as I entered puberty. As I entered adulthood, I suppressed those thoughts because I thought I would change my own self-identity if I ever had sex with a man- I would be leaving the comfortable world of being a member of the majority sexual party- heterosexuals. After my first MM experience I realized it really wasn't a life changing event after all. And more importantly I realized being bisexual was actually different from being gay. So, I began to feel comfortable with how I viewed myself and my sexuality.
It took the greater part of my life to come to this realization. Mostly I was battling the decades of societal shame that was given to anyone engaging in same sex relationships or just having a same sex partner as hugely taboo. So the stats from the OP's post show a generational increase in those identifying as bisexual. It does make me wonder what the contributing factors might be.
My theory is that we now have generations born in this century that have been exposed to the Internet since they were small kids. I think the Internet allows people to search and find just about anything including information on sexual behavior and how people self-identify with some type of sexual orientation. Before the Internet society offered two flavors hetero and homosexual identities. But now there are countless "flavors" being offered up and people are making choices. And more importantly there seems to be an acceptance among the peers within a generation to accept other's choices and not be judgmental thus allowing people to openly choose what feels right for them.
So maybe there always were that many bisexual people out in the world, but now people are much more comfortable admitting it where in the past they were not. I believe the numbers actually are much higher because there are a lot of people who are still afraid to act on their sexual yearnings for fear it could cause fallout with friends, family and co-workers.
Over my lifetime, I think we as a society have come a long way for the better. The more people are free to openly choose a sexual orientation that fits their inner most desires, the less stress they will have. Trying to force fit yourself into a sexual orientation that really doesn't work for you just causes a lot of stress. I think as a society we are quietly moving in the right direction.