The issue comes from the fact that if there's any amount of implantation and the very earliest stages of development, a pregnancy test will come back positive and there are many diagnostic tests (most imaging modalities) and many procedures (such as non-obstetrical emergency surgeries) that will get delayed or sometimes even denied because of the positive pregnancy test. Every AFAB that comes through an ER that might need imaging gets a point-of-care pregnancy test that has to come back negative for something like a CT scan unless it is a very dire situation because of the radiation risk to a developing embryo.

Obstetrical procedures like abortions are not the only ones that are gate-kept for AFAB patients because of the policies surrounding risks to developing embryos or fetuses.

Edit to add: The studies do say that the estimated incidence of clinically unrecognized pregnancies is approximately 20%, but given the myriad risk factors that can feed into that outcome, things like exposures, stresses, SES, and access to medical care may increase that incidence rate in some populations.