Bi isn’t an all or nothing thing. Men are well known to enter same sex relationships when in restricted environments long term such as sailing ships or prison, but not everyone is that flexible.

Women do the same with a lesser known example being elderly women swapping because of the shrinking dating pool.

So, self identifying as bi shouldn’t be taken as having zero gender preferences nor should we assume peoples preferences are static through time. People may be in a committed relationship and seek partners of the a different gender etc.