Er, well, what's happening is a complex social phenomenon. I don't think it's that easy to understand in enough detail to know why something like "assigned sex at birth" might start appearing on forms as opposed to other things.

I do know about birth certificates of course; what I was pointing out is that by asking the assigned sex at birth question, they aren't trying to assess the rates of errors on birth certificates (errors can happen in anything; some states even have explicit instructions for how to fix this type of mistake: https://www.cdph.ca.gov/Programs/CHSI/Pages/Correcting-a-Sex-Field-Error-on-Birth-Certificates.aspx). Or say that we actually think what the birth certificate happens to say is the most important thing. They're not asking the question that way because they want to know the information that this particular question is uniquely poised to answer When it comes to sex, you don't keep going back to your birth certificate to check. If you forget, there are other ways to know (for ~99.97% of people).

I think two things are going on with the AxAB language. The first, and primary, is that it's an authoritative call about what the body looks like. If you just go with "sex", and you don't want to carefully special-case any rare cases where that doesn't fit so well, you have to start getting into arguments about "if your body looks not-dead but you are dead when you're brain-dead, can you be brain-male when your body looks not-male...and if so does sex go with the brain or body in cases where there's a mismatch?" This is a huge can of worms. "Sex at birth" isn't even good enough, because there are already signs that newborn girls' brains are at least on average may not be exactly the same as newborn boys'. Even if the science isn't totally settled, this raises the possibility of being brain-female in a baby male body at birth: you just can't detect it. So even at birth we have to have the is-sex-body-or-brain-if-brains-are-sexed-and-they-can-get-out-of-sync fight.

The other thing is that the LGBTQ+ community has chosen a big-tent strategy. Everyone is in, as long as they are unusual in some aspect of sex or gender or sexual orientation, and some fraction of those people get pushback from others (almost exclusively outside that community) who make more conventional distinctions. It's way easier for that subset if sex is just a matter of declaration...which means at birth the doctor or whoever does the declaration instead of them, hence "assigned sex at birth". Wouldn't that be lovely for avoiding pushback! And there legitimately exist a few very rare cases of ambiguous development where it really was an assignment, so you can point to that and say, "See!" And it clicks with the proclivities of the community. Except...that's...not actually how it works the overwhelming majority of the time. There is a declaration, but it's based on something--and this part is the counterfactual implication that I think you're objecting to.

Fair enough to object, but I really don't think it's a good idea to imply that it's easy to understand what is going on here. From my view it looks like kind of a mess, with even the people proposing the language not really 100% understanding (or agreeing on) what they're doing.