could be someone bisexual who only dates within lesbian spaces and is effectively a lesbian. could be someone genderfluid, bigender, multigender in some capacity who is a lesbian but also gay because they are both a woman and a man (at different times or concurrently). so the person feels the term bisexual also describes them as well as the term lesbian. describing your sexuality when you're non-binary in general, not just multigender, can get weird and complicated. really it's just people who don't feel fully encompassed by the term lesbian but also date within lesbian spaces and add bi on as a sort of modifier to more accurately explain their relationship to their sexuality.