Not the person you were asking, but I can provide an answer. Pansexual generally means attraction to people regardless of gender - sort of gender blind. A bi person (like me) might find that the attraction they experience to different genders is shaped differently, qualitatively — or the magnitude of attraction may be different — like if you were a 1 on the Kinsey scale, which means "predominantly heterosexual, only incidentally homosexual". Someone who's pan is more likely to be a 3 on the Kinsey scale, but also, it's possible to be bi and a 3, and that's subtly different.

There aren't set rules on this, it mostly comes down to what terms resonate with people. I'm someone to whom pansexual as a label could apply, but I identify as bisexual because that was the word that made me go "wait, this is a thing that's possible?". The terms people use are often rooted in history, personal or otherwise.

It's trickier to explain the lexical niche when I myself am not pan. It's like if you're working on a project and have someone passing you tools, and you reach a step that needs a particular spanner, of which you have two. You ask for one of those spanners, but despite it fitting many of your requirements on paper, it isn't quite right for what you're trying to do. You try the other spanner and it's perfect. Keeping both spanners is probably useful because on simple jobs, they are interchangeable, but when getting into nuanced, complex situations, having the choice is useful.

By this, I mean that I have also had the thought that "[Pansexual] seemed a meaningless term because bi already covers basically everything", but when you're talking to someone about different spanners and they say "that one isn't the same as that one. I need the other one", it's generally wisest to assume that this person has some insight that you don't have on these spanners, or their particular use cases — who am I to tell people what tools are most useful for them, after all? Like a lot of identity stuff, it's hard to explain, but it matters a lot to some people.

what the other guy said. it's different for everyone. But some part of me identifying as bi not as pan is also self-reflective. It's because it's the term I want. Like how someone can be Alexander, Alex, Al, Xander, AJ... yet we don't question the difference, we just instinctively understand it's a preference.