Sex as a binary is based upon a lot of different factors, from hormones to chromosomes to secondary sex characteristics to more. For some of those things, people might not know if they don't fit perisex standards for their assigned sex! It's not like testing for, say, chromosomes happens often – and I've heard plenty of horror stories from intersex people whose family and doctors deliberately kept the fact that they were/are intersex a secret from them. Simply put, a person can go all of their life assuming they're perisex while actually being intersex simply due to a lack of information or lack of testing. It's no different than a trans person assuming they're cisgender, or a gay person assuming they're straight – it's all just assumptions based upon the standard that everyone is expected to fit. It's just that finding out if you're intersex is a different process (one that, relevantly, can be blocked or made more difficult by bigoted doctors who aren't interested in helping their patients discover the truth).