Well, the easiest example is that some people use “trans” to mean anyone who has physicslly started to transition, others consider someone to be trans when they decide to broadcast their new gender identity, and others consider them to have always been trans. The opinion on which one is correct is often quite strong.
You could define it as “anyone who says they’re trans is trans” and avoid this entire issue, which is largely what the relevant laws do (unless they’re weirdly invasive), but that opens up the system for abuse by bad actors looking to false flag the trans community.