I have heard several different explanations. Here are the 3 I've seen most often:
1) "Lesbian" is an umbrella term and bisexual women should be considered lesbians.
2) The split attraction model (argues that sexual orientation and romantic orientation are different, so someone can be bi-romantic but homosexual, or vice versa).
3) "Bi" doesn't necessarily mean men and women; it could mean women and nonbinary people, or women and non-men, so someone can like 2 genders but both fall under the sapphic umbrella.
I can't remember hearing the fluid sexuality explanation for this term specifically, but I'm sure it has been used.
I legit can't fathom what a bi-lesbian would be. Like, honestly don't even have a plausible answer.
[Edit: Waitwait, I came up with a guess! Is it someone who's sometimes bi and sometimes lesbian?]
I have heard several different explanations. Here are the 3 I've seen most often:
1) "Lesbian" is an umbrella term and bisexual women should be considered lesbians.
2) The split attraction model (argues that sexual orientation and romantic orientation are different, so someone can be bi-romantic but homosexual, or vice versa).
3) "Bi" doesn't necessarily mean men and women; it could mean women and nonbinary people, or women and non-men, so someone can like 2 genders but both fall under the sapphic umbrella.
I can't remember hearing the fluid sexuality explanation for this term specifically, but I'm sure it has been used.