Just wondering ??

Brittany • Mother to my little man . Wife to his wonderful daddy . 19 .
Why is it that sometimes I'll read these crazy stories about some people not getting there BFP untill like 10 + weeks pregnant & some women don't have a positive their whole pregnancy . I've also read where some people don't get their first positive untill like 21dpo ? 
​I don't understand how it's possible , and then some women can test positive as early as 7-8dpo ?