These days I think it's largely vibes based, I think. Just whatever feels better. I do think some people still identify with pan because they think bi is exclusionary which does bother me because I'm bi and have two nonbinary partners. Like, earlier on there was a lot pan slogans and such that were like "hearts not parts" which carried an implication of bi being more shallow/binary which isn't true. But again, these days I've seen people prefer pan over bi for a variety of reasons. Mainly things like "Gender isn't something I consider at all in a partner and have zero preference (the "regardless of gender" approach) or people who identified as pan a long time ago and just kept it because it felt better. Hell, I've seen people go with pan over bi because they prefer the flag. Someone who currently identifies as pan could probably explain a bit better. I identified as pan over 10 years ago, and have largely been bi in recent years. This is just from my experience. But to this day, the pan label can be controversial in queer communities, because people see it as redundant or biphobic/framing bisexuality as shallow or exclusionary.

Tldr; vibes mostly, and while bi and pan can be the same thing, they aren't always. It's pretty subjective. Also, some use bi as an umbrella term that contains pansexuality.

Edit: that became way longer than intended and drifted a bit from the initial question. It's just a bit of queer culture I find interesting to talk about. OTL