Sex is assigned at birth. What's the margin of error on that do you think?
I'm guessing you weren't a science teacher.
I have taught science, physics but not biology.
Even the term 'sex is assigned at birth' is problematic as Badenoch, for one example, is vocally against the term. The process relies on the one test - can we see a penis? Even if sex is always 100% determined by the presence of a penis, this test still goes wrong.
A few weeks ago I was standing in a queue in the Co-op. A customer was telling the cashier that her few weeks old grandchild was identified at birth as female, before the mother realising after two months that her child appears male when a penis suddenly appeared The birth certificate was already issued. Although certificates can be changed for administrative errors, this was not an admin error; she was worrying about that.
Sex is not only assigned at birth. Trans people with a GRC are given a birth certificate with sex amended. Even then the system fails a bit. In some regions the term 'sex' is used on the birth certificate, in others the term 'gender's is used. These are legacy issues.
There is no available data for us to be able to know precisely the numbers of people born with atypical anatomy / physiology since we just don't test for it. People sometimes only find out that they are atypical as a result of testing as an adult - often being investigated for infertility.